Sweet Potato Layer Cake with Marshmallow Filling
Spiced sweet potato cake layers filled and frosted with a cooked-from-scratch marshmallow fluff. The Thanksgiving marshmallow sweet potato casserole, rebuilt as a proper layer cake.
Servings 12 slices
Ingredients
For the Cake
- 2 ½ cups all-purpose flour spooned and leveled
- 2 ¼ teaspoons baking powder
- 1 teaspoon baking soda
- 1 teaspoon ground cinnamon
- ¼ teaspoon ground nutmeg
- ¾ teaspoon salt
- 6 tablespoons unsalted butter melted and cooled
- 6 tablespoons vegetable oil
- 1 cup granulated sugar
- 1 cup light brown sugar packed
- 3 large eggs room temperature
- 1 ½ cups sweet potato puree roasted and pureed smooth
- ⅔ cup water
- 1 teaspoon vanilla extract
For the Marshmallow Filling
- 1 ½ cups granulated sugar
- 1 ½ cups light corn syrup
- ⅔ cup water
- 6 large egg whites room temperature
- 1 teaspoon cream of tartar
- 2 teaspoons vanilla extract
Instructions
Bake the Cake Layers
- Preheat the oven to 350 degrees F. Butter two 8-inch round pans, line the bottoms with parchment, and butter the parchment.
- Whisk together the flour, baking powder, baking soda, cinnamon, nutmeg, and salt for a full 30 seconds.
- In a large bowl, whisk the melted butter, vegetable oil, granulated sugar, and brown sugar until glossy. Add the eggs one at a time, whisking after each.
- Whisk in the sweet potato puree, water, and vanilla until the mixture is uniform in color.
- Fold the dry ingredients into the wet with a spatula until the last streak of flour disappears. Do not overmix.
- Divide the batter evenly between the pans and bake 35 to 45 minutes, until a skewer comes out clean and the tops spring back.
- Cool in the pans 10 minutes, turn out onto a rack, and cool completely. Wrap and chill for an hour before assembly for cleaner stacking.
Make the Marshmallow Filling
- Place the egg whites and cream of tartar in a spotlessly clean stand mixer bowl. Whip on medium to soft peaks, then stop and hold.
- Combine the sugar, corn syrup, and water in a saucepan over medium-high heat. Clip on a candy thermometer and cook without stirring to 240 degrees F, the soft-ball stage, about 8 to 10 minutes.
- With the mixer running on medium-low, pour the hot syrup down the side of the bowl in a thin, steady stream. Never pour onto the moving whisk.
- Raise the speed to high and whip 7 to 10 minutes, until the bowl feels close to room temperature and the fluff is thick, glossy, and holds a stiff peak.
- Beat in the vanilla for 15 seconds and use the fluff immediately.
Assemble
- Level the cooled layers with a long serrated knife, using a sawing motion and no downward pressure.
- Set the first layer cut side up on a cake stand and spread about 1 ½ cups of fluff to within half an inch of the edge.
- Place the second layer cut side down and press gently, checking that the stack is level from eye height.
- Spread a thin crumb coat of fluff over the whole cake and chill 20 minutes until tacky.
- Finish with the remaining fluff, spread in big swoops with an offset spatula. Toast the peaks with a kitchen torch held 4 inches away if desired.
Notes
- Corn syrup is required: It prevents the sugar from crystallizing, which is what keeps the fluff smooth instead of grainy.
- Spotless bowl: Any trace of fat or yolk stops the egg whites from whipping. Wipe the bowl and whisk with vinegar if unsure.
- Hit 240 degrees F: A syrup that falls short leaves the meringue unstable and it will weep.
- Three-layer option: Use three 6-inch pans and reduce the bake time by about 5 minutes.
- Sheet cake option: Bake the full batter in a 9x13-inch pan at 350 degrees F for 40 to 48 minutes and halve the filling.
- Jarred shortcut: Whip 1 cup softened butter with one 7 oz jar marshmallow creme, 2 cups powdered sugar, 1 teaspoon vanilla, and 1 to 2 tablespoon milk.
- Storage: Loosely covered at room temperature up to 2 days, or refrigerated up to 4 days. Do not freeze the assembled cake.