Cook the elbow noodles in plenty of boiling salted water until al dente, 10-12 minutes. Drain. Return cooked macaroni to saucepan and stir in the cream. Bring to a boil, then reduce heat and simmer for 6 minutes. Remove pan from the heat and stir in 2/3 of the cheese, the nutmeg, salt, and pepper. Preheat the broiler. Transfer the macaroni mixture to a gratin dish and sprinkle the remaining cheese on top. Sprinkle the butter over the surface. Broil until the gratin is bubbling and golden brown, 5-10 minutes.
