Place the potatoes in a large pot. Add cold water to cover and bring to a boil. Add 2 teaspoons salt, reduce heat, and simmer until tender, 15 to 18 minutes. Drain the potatoes and return them to the pot.

Add the sour cream, milk, butter, scallions, 1 teaspoon salt, and 1/4 teaspoon pepper and mash. Transfer to a heatproof bowl, cover, and set over a pot of simmering water to keep warm, up to 1 hour.