There's a piping tip in my drawer that cost eleven dollars and does exactly one thing.
It's a flat little disc drilled with about a dozen pinholes, and it looks like a shower head that shrank in the wash.
I bought it after standing in front of a bakery case in Kyoto for what my wife swears was four minutes.
Behind the glass sat these squat domes wrapped in what looked like violet yarn.
Hundreds of thread-thin strands, wound around and around, dusted white on top like a peak in snow.
That's the whole idea.
Mont Blanc is a French dessert named after the mountain, and it's classically made with chestnut.
Japan borrowed it decades ago and made it their own, and every autumn the chestnut gets swapped for beni imo, the purple sweet potato.
I went home and made a mess of it.
My first three batches came out as short broken worms instead of long strands, and I blamed the tip.
The tip was fine.
I hadn't pushed the cooked sweet potato through a sieve, so the fibers were plugging those pinholes from the inside.
Once I sorted that out the strands came off the nozzle in one long ribbon, and have every time since.
It's a fussy-looking dessert that turns out to hinge on one boring step.
I promise the sieve is the whole trick!
What a Mont Blanc Actually Is
This dessert is French by birth and Japanese by adoption. Both halves of that matter, and most recipes skip straight past it.
The original comes from the Alps, from the border country between France and Italy, and it's named for Mont Blanc itself.
Sweetened chestnut puree gets piped in thin strands over whipped cream on a small base, then dusted with powdered sugar.
The strands are the snow-streaked slopes and the sugar is the snow cap.
It's one of the few classic desserts that is literally a picture of something.
Japan picked it up in the early twentieth century and it became a fixture in Japanese patisserie, where it's arguably more popular now than it is in France.
Somewhere along the way Japanese bakers started making it with satsumaimo and then with beni imo, the deep purple sweet potato from Okinawa.
Autumn is sweet potato season there, so the purple version shows up in bakery cases from about September through November.
Is it traditional? Not in France. It absolutely is in Japan.
The flavor shift is real and worth knowing before you commit. Chestnut is nutty, a little tannic, almost savory at the edges.
Purple sweet potato is milder, gently floral, and sweeter, with a texture that's silkier once it's sieved.
And the color is outrageous. Nothing else in a home kitchen turns that shade of violet without a bottle of dye.
Ingredients You'll Need
Short list, and the sweet potato is doing almost all of it. Everything else is there to make the puree pipeable and to give it something to sit on.
- Purple sweet potato (1 pound, about 2 medium). The whole dessert. Look for beni imo or Okinawan sweet potato first, purple skin and vivid purple flesh. Stokes Purple is the common US supermarket version and works well.
- Unsalted butter (3 tablespoons, softened). Richness, and it keeps the paste supple so it extrudes instead of crumbling.
- Sweetened condensed milk (3 tablespoons). This is the one that surprised me. It sweetens without adding water, which plain syrup would, and water is what makes a paste sag off the nozzle.
- Granulated sugar (2 tablespoons). Just enough to back up the condensed milk. Purple sweet potato is less sweet than the orange kind.
- Heavy cream (2 to 4 tablespoons, for the paste). Added a spoonful at a time to hit piping consistency. Start low.
- Dark rum (1 teaspoon, optional). Classic in Japanese Mont Blanc and it cuts the sweetness. Leave it out and nobody will file a complaint.
- Salt (⅛ teaspoon). Sweet potato goes flat without it.
- Egg white (1 large) and granulated sugar (¼ cup). For the meringue bases. One egg white makes four discs with room to spare.
- Heavy cream (¾ cup), powdered sugar (1 tablespoon), vanilla (¼ teaspoon). The cream dome under the strands. It has to be whipped stiff enough to hold a shape.
- Powdered sugar, for dusting. The snow cap. Non-negotiable.
Choosing Your Purple Sweet Potato
Nobody warns you about this one: ube is not purple sweet potato. I bought ube extract for my second attempt thinking I was being clever.
Ube is a yam. Different plant, different family, vanilla-ish and much more perfumed.
Purple sweet potato is a sweet potato with anthocyanin-rich flesh, and it tastes like a sweet potato.
- Beni imo. The Okinawan one, the most authentic choice. Pale tan skin, brilliant violet flesh, dry and starchy. Japanese and Korean groceries carry it in fall.
- Okinawan sweet potato. Same thing under a different label in most US markets. Grab it if you see it.
- Stokes Purple. Purple skin and purple flesh, widely available at Whole Foods and Trader Joe's. Denser and drier than beni imo, so it needs an extra spoonful of cream.
- Murasaki. Purple skin, white flesh. Delicious, but it will give you a beige Mont Blanc. Skip it for this.
How to Make Purple Sweet Potato Mont Blanc
You're building three separate things and then stacking them. The bases and the paste both keep, so this splits across two days without any loss.
Step 1: Steam and Sieve the Purple Sweet Potato
Peel the sweet potatoes and cut them into 1 inch chunks.
Steam them over simmering water for 18 to 22 minutes, until a knife slides through with no resistance at all.
Steam, don't roast. A dry oven heat caramelizes the sugars and drags the color from violet toward a muddy brown, and the color is why you're here.
Pro tip: boiling works too, but drain them and return them to the dry hot pot for a minute to drive off the surface water.
Now the important part.
Press the hot flesh through a fine-mesh sieve with the back of a spoon or a stiff silicone spatula.
Then press it through a SECOND time.
Purple sweet potato carries fine threads of fiber that you'll never see and never taste, and every one of them is exactly the right size to jam a pinhole in your piping tip.
A food processor chops those fibers into shorter fibers. It does not remove them.
Step 2: Make the Mont Blanc Paste
Beat the warm sieved sweet potato with the softened butter until it disappears into the puree.
Add the condensed milk, the 2 tablespoons of granulated sugar, the salt, and the rum if you're using it.
Now add heavy cream one tablespoon at a time, beating between each one.
You're looking for the texture of a thick, smooth frosting that holds a ridge but sags slightly at the tip.
Too stiff and it will snap into fragments. Too loose and the strands melt together into a purple blob.
Chill the paste for 30 minutes before piping. Cold paste holds its thread.
Step 3: Bake the Meringue Bases
Heat the oven to 225°F.
Whip the egg white with a pinch of salt to soft peaks, then add the ¼ cup of sugar a spoonful at a time until it's glossy and stiff.
Pipe or spoon four flat discs about 2 ½ inches across onto parchment.
Bake for 75 to 90 minutes, until they lift off the paper cleanly and sound hollow when tapped.
Cool them completely in the turned-off oven with the door cracked.
Want a shortcut? A store-bought mini tart shell or even a plain sponge round does the job. The base is structural, not the point.
Step 4: Whip and Dome the Cream
Whip the ¾ cup of heavy cream with the powdered sugar and vanilla to firm peaks.
Stiffer than you'd want on a slice of pie. It has to support a nest of strands without slumping.
Mound a rounded dome of cream onto each meringue disc, taller than it is wide, and smooth it into a cone with a small offset spatula.
Chill the assembled domes for 15 minutes while you fit the piping bag.
Step 5: Pipe the Strands
Fit a piping bag with the Mont Blanc tip, the flat one with the row of tiny holes, and fill it no more than two-thirds full.
Start at the base of the cream dome and pipe with steady, even pressure while you rotate the plate underneath.
Wind the strands up and around the dome, overlapping as you climb, until the cream is completely hidden.
Don't stop and restart if you can help it. One continuous pass looks like a bakery did it.
Finish with a snowfall of powdered sugar over the top.
Serve within a couple of hours. Yum!
Tips for Clean, Unbroken Strands
Every failure I've had with this dessert came down to one of these six things. Run through them before you fill the bag, not after.
- Sieve twice. Really. This is the difference between strands and crumbs, and it takes four minutes. Fiber in the paste is the number one cause of a clogged tip.
- Test the consistency on a plate first. Squeeze a short length onto a saucer. If it breaks within an inch it's too dry, so add cream. If it flattens and spreads, chill it longer.
- Chill the paste, and chill the bag if your kitchen is warm. Butter softening in your hand as you squeeze will loosen the paste mid-pipe.
- Even pressure beats fast hands. Squeeze from the top of the bag with a constant grip and let the plate do the moving. Pulsing the pressure is what snaps a strand.
- Don't overfill the bag. More than two-thirds and you can't control the pressure, and the paste warms faster from the heat of your palm.
- When a strand snaps, keep going. Lay the next pass over the break. Nobody has ever spotted a repair in a finished Mont Blanc.
No Mont Blanc Tip? Three Ways Around It
You do not need an eleven dollar single-purpose nozzle to make this. I used all three of these before mine arrived.
- A potato ricer. The best substitute by a mile. Load it with chilled paste, hold it directly over the cream dome, and press slowly while you turn the plate. The strands come out slightly thicker and it looks fantastic.
- A small plain round tip. A number 2 or 3 round tip piped in long overlapping lines up and over the dome. Slower, and it reads as ribbons instead of vermicelli, but it's genuinely pretty.
- A snipped piping bag. Cut the smallest opening you can and pipe in repeated passes. This is the roughest of the three, so lean into it and pipe a loose messy nest on purpose.
Make Ahead and Storage
Every component holds well, and the assembled dessert holds badly. Plan around that and you're fine.
- The sweet potato paste. Up to 3 days in the fridge, covered tight. Let it sit out for 15 minutes and beat it briefly before piping, since it firms up cold.
- The meringue bases. A full week in an airtight tin at room temperature. Never the fridge, humidity turns them chewy overnight.
- The whipped cream. Best the day of. It weeps by day two and takes the strands down with it.
- Assembled Mont Blancs. Two hours, maybe three. The meringue starts pulling moisture from the cream and softens from the bottom up. Assemble right before you serve them.
- Freezing. The paste freezes well for 2 months. Thaw overnight in the fridge and beat in a spoonful of cream to bring it back.
Fussy-looking, honestly not that fussy. Give it a try!
Recipe Card
Recipe
Purple Sweet Potato Mont Blanc
Ingredients
For the Meringue Bases
- 1 large egg white room temperature
- ¼ cup granulated sugar
- 1 pinch salt
For the Purple Sweet Potato Paste
- 1 lb purple sweet potato beni imo, Okinawan, or Stokes Purple, about 2 medium
- 3 tablespoon unsalted butter softened
- 3 tablespoon sweetened condensed milk
- 2 tablespoon granulated sugar
- 2 to 4 tablespoon heavy cream added a spoonful at a time
- 1 teaspoon dark rum optional
- ⅛ teaspoon salt
For the Cream and Finish
- ¾ cup heavy cream cold
- 1 tablespoon powdered sugar
- ¼ teaspoon vanilla extract
- 2 teaspoon powdered sugar for dusting
Instructions
Bake the Meringue Bases
- Heat the oven to 225°F and line a sheet pan with parchment. Whip the egg white with a pinch of salt to soft peaks, then add the ¼ cup granulated sugar a spoonful at a time and whip until glossy and stiff.
- Pipe or spread four flat discs about 2 ½ inches across. Bake for 75 to 90 minutes, until they lift cleanly from the parchment and sound hollow. Cool completely in the turned-off oven with the door cracked.
Make the Purple Sweet Potato Paste
- Peel the purple sweet potatoes and cut into 1 inch chunks. Steam over simmering water for 18 to 22 minutes, until a knife meets no resistance. Steam rather than roast, as dry oven heat dulls the violet color.
- Press the hot flesh through a fine-mesh sieve, then press it through a second time. This removes the fine fibers that clog a Mont Blanc piping tip and is the single most important step in the recipe.
- Beat the warm sieved sweet potato with the softened butter until smooth, then beat in the condensed milk, granulated sugar, salt and rum. Add heavy cream one tablespoon at a time until the paste holds a ridge but sags slightly at the tip.
- Chill the paste for 30 minutes.
Assemble
- Whip the ¾ cup heavy cream with the powdered sugar and vanilla to firm peaks. Mound a tall rounded dome of cream onto each cooled meringue disc and smooth it into a cone. Chill for 15 minutes.
- Fit a piping bag with a Mont Blanc tip and fill it no more than two-thirds full. Starting at the base of each dome, pipe with steady even pressure while rotating the plate, winding the strands up and over until the cream is completely covered.
- Dust generously with powdered sugar and serve within a couple of hours.
Notes
- Sieve twice: Fine fiber in purple sweet potato clogs the piping tip and breaks the strands. A food processor shortens the fibers, it does not remove them.
- Steam, do not roast: Oven heat caramelizes the sugars and turns the violet flesh brownish.
- Ube is not purple sweet potato: Ube is a yam with a different flavor. Use beni imo, Okinawan, or Stokes Purple sweet potato.
- No Mont Blanc tip? A potato ricer held over the cream dome is the best substitute.
- Storage: Paste keeps 3 days refrigerated, meringues keep 1 week airtight at room temperature, but assembled Mont Blancs soften within a few hours. Assemble to order.
