For the frying, the flavor of regular olive oil, which is also cheaper, is preferable to that of extra-virgin. Make these to serve with our Mushroom-and-Bacon Dip.
Using a mandoline fitted with the zigzag blade, firmly and evenly slice potatoes 1/8 inch thick, rotating a quarter-turn after each slice to create a waffle pattern. Soak slices in cold water overnight.
Fill a heavy-bottomed pot with 2 inches oil. Heat over medium-high until a deep-fry thermometer reads 340 degrees. Meanwhile, drain potatoes; dry well with paper towels.
Fry potatoes in batches, turning occasionally, until golden brown and bubbles subside, about 3 minutes a batch. (Adjust heat between batches to keep oil temperature between 325 degrees and 340 degrees.) Drain on paper towels. While still warm, season with salt.