Ingredients

• 5 lb yellow-fleshed potatoes such as Yukon Gold, peeled and cut into 1-inch pieces
• 1 tablespoon salt
• 1 cup whole milk
• 1/2 cup sour cream
• 1 1/2 sticks (3/4 cup) unsalted butter
• 2 bunches scallions, cut crosswise into 1/2-inch pieces
• 1/2 teaspoon black pepper
• Special equipment: a potato masher or a food mill fitted with medium disk

Instructions

1. 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.
2. 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.
3. 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.
4. 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.