Double-frying is the secret of both perfect french fries and beautiful pommes soufflées. Just the cuts and varying oil temperatures make them different.
4 lbs. Idaho potatoes, peeled
1. Cut potatoes into 4'' lengths about 1/4'' x 1/4'' x 3'' (it is easiest to do this with a mandoline). Place in a large nonreactive bowl, cover with water, and set aside in the refrigerator for about 2 hours.
2. Heat about 4'' oil in a heavy-bottomed pot over medium heat. Drain potatoes, then dry thoroughly with paper towels. Check oil temperature with a thermometer; when oil reaches 325°, cook potatoes, without browning, in small batches, turning occasionally, until they are tender and their edges are slightly crisp, about 4 minutes. Drain potatoes on paper towels and allow to cool for about 20 minutes. (Potatoes may be cooked to this point several hours in advance of serving and kept in refrigerator until ready to crisp.)
3. Reheat oil. When temperature reaches 375°, fry potatoes in small batches until crisp and golden, 1–2 minutes per batch. Drain on paper towels, sprinkle with salt, and serve.
POMMES SOUFFLÉES: Making pommes soufflées is tricky. To puff, they must be the proper thickness and neither too big nor too small. Even armed with a mandoline (an absolute necessity), these take practice. To begin, shape whole potatoes into rectangles or cylinders about 2 1/2'' long and about 1 1/2'' wide, then slice potatoes evenly, 1/8'' thick. Place slices in a nonreactive bowl and cover with water. Proceed as for french fries in steps 2 and 3 above, but heat the oil for the initial cooking to only 250°, and for the second cooking to 400°. (Potatoes should blister slightly during the first fry and puff immediately during the second.) Pommes soufflées should be cooked until crisp. Serve immediately or recrisp for 20 seconds in 400° oil.