In a medium sauce pan combine cornstarch, salt, dry mustard, and pepper. Slowly whisk in milk until smooth. Add butter and to a boil over medium heat until it thickens slightly, stirring constantly. Take care not to scorch the milk. Remove from heat.
Reserve 1/4 cup grated cheese for topping. Whisk remaining cheese into the milk mixture until smooth.
Drain macaroni and return to pot. Stir in cheese sauce and poor into a 9x13 casserole dish. Sprinkle with remaining cheese and bake for 25 minutes or until hot and bubbly.