Place the potatoes in a large pot and add enough cold water to cover. Add 2 teaspoons salt and bring to a boil. 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, 1 teaspoon salt, and ¼ teaspoon pepper to the potatoes and mash to the desired consistency. Sprinkle with the scallions.