Peel the potatoes, cut in half, and place in a pan, then cover with cold salted water. Bring to a boil and continue cooking until the potatoes fall off the fork when pierced, about 20 minutes. Drain the potatoes. Gently press the potatoes through a sieve (or potato ricer) into a large bowl, using a rubber spatula.
Mix in the butter, milk, sour cream, salt and pepper and stir to blend. Add a tablespoon or 2 more of warm milk, if necessary to obtain the desired consistency. Salt and pepper liberally and serve.