3 pounds boiling potatoes, such as Yukon gold, peeled
1/2 cup milk
1/4 pound (1 stick) unsalted butter
3/4 to 1 cup buttermilk
1/2 teaspoon freshly ground black pepper
In a large pot, bring 4 quarts of water and 2 tablespoons of salt to a boil. Meanwhile, cut the potatoes into 1 1/2-inch cubes and add them to the boiling water. Bring the water to a boil again, lower the heat and simmer uncovered for 10 to 15 minutes, until the potatoes fall apart easily when pierced with a fork.
Meanwhile, heat the milk and butter in a small saucepan, making sure it doesn't boil. Set aside until the potatoes are done.
As soon as the potatoes are tender, drain them in a colander. Place a food mill fitted with a small disc/blade over a heatproof bowl. Process the potatoes through the food mill, turning the handle back and forth. As soon as the potatoes are mashed, stir in the hot milk-and-butter mixture with a whisk or rubber spatula. Add enough buttermilk to make the potatoes creamy. Add 2 teaspoons of salt and the pepper, to taste, and serve hot.