Heat oven to 300° F. Place the potatoes in a large pot with enough cold water to cover them by 2 inches. Bring to a boil and cook until tender, about 15 minutes. Drain well, return to pot, and mash. Add 8 tablespoons of the butter, the cream, milk, salt, and pepper. Transfer to a casserole and dot with the remaining butter, cut into small pieces. Place in oven, uncovered, to keep warm for up to 1 hour.
In Advance: For a crisp, golden crust on the potatoes, increase the oven temperature to 400° F for the last 15 minutes.