How to Make It
Cut and discard 1/2 inch from ends of potatoes. Cut potatoes in half lengthwise; cut halves crosswise into quarters. Slice quarters lengthwise into 1/4-inch-thick slabs.
Place potatoes in a large bowl; rinse under cold running water until water runs clear. Drain potatoes and place in a stock pot; add 8 cups water, vinegar, and kosher salt.
Set pot over medium-high and bring to a simmer. Reduce heat and simmer until potatoes are just tender enough to be pierced with a fork, about 15 minutes.
Using a spider or slotted spoon, carefully lift potatoes from water and arrange in a single layer on a wire rack set inside a rimmed baking sheet. Let stand until cool.
In a large heavy pot, heat 2 inches of oil to 250°F. Working in batches, fry potatoes until tender, about 6 minutes. Lift potatoes from oil and place on rack to drain.
When potatoes are cool, arrange on a parchment paper—lined baking sheet and freeze. (Potatoes can be kept frozen up to 1 month in an airtight container.)
Heat 2 inches of oil to 350°F. Working in batches, fry frozen potatoes until puffed and golden, 3 to 5 minutes. Remove fries and place on paper towels to drain.
Place hot fries in a large bowl. Sprinkle with flaky sea salt, and toss well to coat. Serve immediately with XO sauce, aioli, spicy mayo, or your favorite dip for dunking.