Cut the potatoes into 1-inch cubes and place them in a large pot. Cover the potatoes with cold water and add enough salt so the water tastes quite salty. Bring to a boil, then lower the heat and simmer, uncovered, for about 10 to 12 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 glass bowl. Process the potatoes through the food mill, turning the handle back and forth to force the potatoes through the disc. As soon as the potatoes are mashed, slowly whisk in enough of the hot milk/butter mixture to make the potatoes very creamy. Add 2 teaspoons of salt and the sour cream and pepper and whisk to combine. Taste for seasoning and serve hot.