Position rack in bottom third of oven; preheat to 400 degrees F. Butter a 11x7 inch glass baking dish. Cook macaroni in large pot of boiling salted water until tender but still firm to bite. Drain well.
Whisk 1/2 cup Gruyere, cream, milk, prosciutto, Parmesan and nutmeg in large bowl to blend. Add macaroni and toss to coat. Season with salt and pepper. Transfer to prepared baking dish. Sprinkle remaining 1 cup Gruyere over. Bake until cheese melts and macaroni and cheese sets, about 20 minutes. Serve warm.