Boil macaroni in water until tender and drain thoroughly. Stir in butter and egg. Mix mustard and salt with 1 tablespoon hot water and to milk. Add cheese, leaving enough to sprinkle on top. Pour into buttered casserole dish, add milk, and sprinkle with the remaining cheese. Bake at 350 degrees for about 40 minutes or until custard is set and top is crusty.
(This is a family favorite and this was former President Reagan's favorite recipe for Homemade Mac & Cheese.)