Peel the potatoes and cut into 1 inch pieces. Place in a large pot and cover with cold water. Add enough salt so that the water tastes slightly salty. Cover the pot, bring to a boil, lower the temperature and simmer for 12 minutes or until the potatoes are fork tender. Drain the water from the potatoes, place the pot back on the stove and cook, stirring, for a minute or two to cook off any excess water. Mash the potatoes with a potato masher or handheld mixer.
While the potatoes are cooking, combine ½ cup milk and butter in a small saucepan. Heat over low heat just until the butter is melted and the mixture begins to simmer. Do not let boil.
Add the sour cream to the mashed potatoes and whip. Add the heated milk mixture to the potatoes and whip. If needed, add more milk, a little at a time, and whip until a light, creamy texture is achieved. Season to taste with salt and pepper. Serve warm.