Preheat oven to 375 degrees.

Cook macaroni according to instructions. Drain.

In medium saucpan, melt butter.

Stir in flour, mustard, and salt. Gradually stir in milk. Cook and stir until mixture thickens slightly.

Remove from heat.

Add 1 cup cheese to milk mixture and stire until melted.

Stir in macaroni.

Turn into greased 2 quart baking dish.

Top with remaining cheese.

Bake 20-25 minutes or until bubbly.