President Reagan's Favorite Macaroni and Cheese

8 ounces macaroni
1 teaspoon butter
1 egg, beaten
3 cups grated sharp cheese
1 teaspoon dry mustard
1 teaspoon salt
1 tablespoon hot water
1 cup milk

Cook macaroni in boiling water until tender; drain thoroughly. Stir in butter and egg. Add cheese reserving enough to sprinkle on top. Pour into buttered casserole.

Mix mustard and salt with hot water and add to milk. Pour over macaroni; sprinkle with remaining cheese. Bake at 350 degrees F for about 45 minutes or until custard is set and top is crusty.