How to Make It
Place the potatoes in a large pot and add enough cold water to cover. Bring to a boil and add 2 teaspoons salt. Reduce heat and simmer until very tender, 20 to 25 minutes. Drain the potatoes and return them to the pot.
Add the butter, sour cream, ¾ cup of the milk, 1 teaspoon salt, and ½ teaspoon pepper to the potatoes and mash until smooth (adding more milk if necessary). Serve topped with additional butter and pepper.