How to Make It
Preheat the oven to 400°. Butter a 9-by-13-inch baking dish. Put the potatoes in a large saucepan and cover with water. Salt the water and bring to a boil, then simmer the potatoes over moderate heat until tender, about 12 minutes. Drain the potatoes, return to the saucepan and shake over high heat for 1 minute to dry. Pass the potatoes through a ricer into a large bowl. Beat in the milk and 6 tablespoons of the butter and season with salt and pepper.
In a large stainless steel bowl, whip the cream to soft peaks. Beat one-third of the cream into the potatoes, then fold in the remaining cream. Scrape the potatoes into the prepared dish. Dot with the remaining 1 tablespoon of butter and sprinkle the Parmesan over the top. Bake the potatoes for 25 minutes. Preheat the broiler and broil the potatoes for 2 minutes, or until browned. Let stand for 10 minutes before serving.