Brown Butter and Scallion Mashed Potatoes

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