Cook macaroni until done. Drain.
Place macaroni, butter, salt, milk, both cheeses in a casserole dish. Mix egg in a bowl first then stir into the casserole dish with the rest of the ingrediants.
Cover casserole dish with aluminum foil and cook for 30 minutes at 350 degrees.
Uncover and if desired, add extra cheese across the top. Bake uncovered for 10-15 minutes.