Working with 1 potato at a time, cut 1⁄4" off each side to square the potato into a rectangle. Rest potato on a flat side. Using a paring knife, trim the potato into the shape of a football. Using a mandoline or a sharp knife, slice potatoes lengthwise to 1⁄8" thickness and transfer to a bowl filled with cold water.
Set two 6-qt. heavy-bottomed dutch ovens over 2 burners. Pour oil to a depth of 2" into both. Heat the first over medium-high heat until a deep-fry thermometer reads 320˚ and the second until a deep-fry thermometer reads 400˚. Drain potatoes and pat dry. Working in batches, add potatoes to the first pot and cook, maintaining a temperature of 275˚ (adding potatoes will cause temperature to drop) and flipping potatoes occasionally with a slotted spoon until they're blistered and translucent, about 5 minutes. Working in batches, transfer potatoes to the second pot and cook, flipping occasionally with a spoon and maintaining a temperature of 375˚, until potatoes are puffed and browned, about 5 minutes. Transfer pommes soufflés to a rack set over a rimmed baking sheet; season with salt and serve.