Use equal parts butter, cream, and fresh grated parmesan. Melt the butter in a saucepan over low or medium heat, whisk in the cream, and let simmer for 5-10 minutes until it thickens a little. Stir in the cheese and some salt and pepper to taste (if you used salted butter, you probably won't need any more salt).
I love to serve this over noodles with grilled chicken. It isn't very good once it cools (so don't save any for leftovers), but it will keep warm in a crock pot for hours without separating.