Put the potatoes in a large saucepan, cover with cold water by about 1 inch and add 2 tablespoons salt. Bring to a boil, uncovered, over high heat, and then reduce the heat to medium-high and simmer briskly until fork tender, about 45 minutes. Drain the potatoes. Quarter the potatoes and pass them through a food mill or ricer back into the pan. (Peel them first if you are using a ricer).
When the potatoes are almost done, warm the cream in a small saucepan over medium-high heat to just under a simmer, about 5 minutes. Whisk in the cheese until melted. Set aside.
Fold the hot cheese and cream into the potatoes. Then, over low heat, fold in the butter bit by bit until fully incorporated. Season with salt and pepper and serve hot.