Combine corn starch, salt, dry mustard and pepper in a medium saucepan. Stir in milk until smooth. Add margarine. Stirring constantly, bring to a boil over medium-high heat and boil for 1 minute. It's very important that you stir constantly so as not to burn, do not skip this step! After the mixture has boiled for 1 minute, remove from heat. Reserve 1/4 cup cheese for topping. Stir in remaining cheese until melted. Add elbow macaroni noodles to cheese mixture and stir to combine. Turn into greased 2-quart casserole dish. Sprinkle with reserved cheese. Bake uncovered in 375 degree oven for 25 minutes or until hot and bubbly.