Cover potatoes with cold water by 2 inches in a 5- to 6-quart heavy pot and add 1 1/2 teaspoons salt. Bring to a boil, then reduce heat and simmer, uncovered, until potatoes are tender, 10 to 15 minutes.

Drain potatoes in a colander, then return to pot and cook over moderate heat, shaking pot occasionally, until dry, 1 to 2 minutes. Mash hot potatoes with masher or force through food mill into another pot, then cover to keep warm.

Bring milk with remaining 1 1/2 teaspoons salt just to a boil in a 1-quart saucepan, then remove from heat and stir into potatoes until combined. Stir in sour cream, then cover and keep warm.

Cook butter in a 12-inch heavy skillet over moderately high heat, stirring frequently and scraping up brown bits, until foam subsides and butter is golden brown, about 8 minutes. Add scallions and pepper and cook, stirring, until slightly softened, about 2 minutes. Fold butter mixture into mashed potatoes and season with salt.