POMMES SOUFFLÉES: Making pommes soufflees 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 soufflees should be cooked until crisp. Serve immediately or recrisp for 20 seconds in 400° oil.