Heat the oven to 425F and let it run a full 20 minutes past the preheat signal so the pans and the air are genuinely at temperature.

Slice a thin strip off one long side of each sweet potato so it sits flat. Cut the potatoes lengthwise into ¼ inch planks, stack the planks, and cut them into ¼ inch sticks. Keep the thickness even.

Spread the cut fries on a clean kitchen towel, pat them dry, and let them air dry a few minutes until the surfaces look matte rather than shiny.

Put the dry fries in a large bowl, add the cornstarch, and toss until you see a faint even dusting. Then add the olive oil and toss again until every stick is glossy. Do not add salt or spices yet.

Divide the fries between two rimmed sheet pans, directly on the bare metal with no parchment or foil, in a single layer with space around each fry.

Bake 18 to 20 minutes without touching them. The crust needs to set and release from the pan on its own.

Flip each fry, swap the pans between the racks, and bake another 8 to 12 minutes, until the edges are deep brown and the fries look shrunken and rigid.

Toss the hot fries right on the pan with the salt, garlic powder, smoked paprika, black pepper, and cayenne. Serve within a few minutes, moving them off the hot pan if they will sit at all.