Cook macaroni as directed on package. Melt butter; stir in flour. Make a smooth paste. Add milk, stirring often. Cook until thickened. Add powdered cheese to milk sauce. Add drained macaroni to sauce; stir. Put macaroni and sauce in greased 9x13 inch glass baking dish. Sprinkle with grated cheese; dot with 2 tablespoons butter. Bake at 350 F for 15 to 20 mintues. Place under broiler for a few minutes for a brown crust on top.