Wash the potatoes and peel them, using a swivel-bladed paring knife. Drop them as they are peeled into cold water.
Cut the peeled potatoes in quarters and place them in a large saucepan. Add water to cover. The water should be about one inch above the top of the potatoes. Add salt. Bring the potatoes to a boil and simmer until they are tender but not mushy. Put the hot potatoes through a food mill or potato ricer. While they are still hot, beat in the butter with a wooden spoon.
Beat in the hot milk and cream, a little at a time. Add salt and pepper to taste and serve hot.