Peel the potatoes and slice as thinly as possible on a mandoline directly into a large, 8-quart container filled with 4 quarts of cold water. Cover and refrigerate overnight.
Drain the potatoes and rinse with clean, cold water; spin dry in a salad spinner. Transfer the potatoes to an 11-quart pot, cover with the milk and set over medium heat. Bring to a simmer, about 35 minutes. Reduce the heat in order to maintain a simmer and cook, uncovered, until the potatoes are fork tender and beginning to fall apart, 25 to 30 minutes.
Reserve 1 cup of the cooking milk. Drain the potatoes thoroughly in a colander and return them to the pot. Press the potatoes through a ricer into a large mixing bowl. Add the reserved hot milk, butter and salt and use an electric hand mixer to whip on low speed for 15 to 30 seconds. Do not overwhip.
Recipe courtesy of Alton Brown, 2011