Heat the oven to 425°F. If desired, place two large rimmed baking sheets in the oven as it preheats so the fries begin cooking on hot surfaces.

Scrub the sweet potatoes well and peel them, or leave the skins on. Cut them into evenly sized fries about ¼ inch thick.

For a crisper result, soak the cut fries in cold water for 30–60 minutes. Drain them and dry them extremely well with clean kitchen towels or paper towels.

Toss the thoroughly dried fries with 1½ tablespoons cornstarch until the coating is light and even. Add 2 tablespoons oil, smoked paprika, garlic powder, kosher salt, black pepper, and cayenne if using, then toss again.

Arrange the fries in a single layer on the baking sheets, leaving space between each piece. Use two sheets if necessary because touching or overlapping fries will steam instead of browning.

Bake for 15–18 minutes. Turn the fries carefully with a spatula so the second side can brown evenly.

Bake for another 10–17 minutes, watching closely near the end, until the fries are browned at the edges and tender inside. Remove them from the oven and season with a little additional salt while hot.

Serve immediately for the crispest texture. Sweet potato fries naturally soften as they stand, so bring them to the table as soon as they are seasoned.