Boil the box of macaroni in large pot of lightly salted water till nearly done(10-12 min) drain and rinse with hot water, set aside.
In a med saucepan make a white sauce by melting the butter (may use less if desired but at least 3 tablspoons), slowly add the flour to the butter and stir into the butter as you slowly heat, keep stirring and watching this as you add enough flour to get a nice thick paste, do not brown, now add the milk and salt pepper, and dry mustard and wisk or stir continously over med high heat till a slight boil. when this has thickened , add 2 cups of grated cheese and fold together with the drained macaroni, Pour this mix into a buttered 2 qt. casserole dish, next sprinkle the top with the extra cup og grated cheese and if desired a few bread crumbs. salt and pepper. Bake in preheated 350 degree oven till cheese is bubbly and golden, (about 35 min)