In saucepan, melt butter. Blend in flour, salt, and mustard. Add milk, stir constantly until sauce thickens a little. Add 1 1/2 c. cheese, heat until melted, stirring.
Remove from heat.
Meanwhile, cook macaroni as directed, drain. Combine with sauce in a buttered 2 quart casserole, top with remaining cheese, then buttered bread crumbs and sprinkle with paprika.
Bake at 375 F about 20-25 minu or until nicely browned.