Preheat oven to 375° F. In medium sauce pan melt butter and stir in flour, mustard and salt. Gradually stir in milk. Cook and stir until mixture thickens slightly (mixture should coat spoon).
Remove from heat. Add 1 1/2 cup cheese, stir until melted. Stir in cooked macaroni. Turn into greased 1 1/2 quart baking dish. Top with remaining cheese. Bake 20 to 25 minutes or until bubbly.
Servings: