Peel the potatoes, cut them into 1-inch cubes and place them in a large pot of boiling salted water. Simmer uncovered for 10 to 12 minutes, until the potatoes are tender. Drain the potatoes in a colander and then return them to the pot.
Meanwhile, heat the milk and butter in a small saucepan until the butter is melted.
Using a hand held mixer, break the potatoes up and at low speed slowly add the hot milk/butter mixture to make the potatoes very creamy. Add the sour cream, 2 teaspoons of salt and the pepper and whisk to combine. Serve hot.