In medium saucepan, combine cornstarch, salt and pepper; using wire whisk, stir in milk until smooth. Add butter. Stirring constantly with rubber spatula, bring to boil over medium-high heat and boil 1 minute. Remove from heat.
2Stir dry mustard into white sauce; add 1 3/4 cups grated cheese and stir until melted (reserve 1/4 cup cheese for topping). Cook elbow macaroni for six minutes; drain. Add cheese sauce to macaroni; stir. Turn into a greased, 2-quart casserole dish; stir in ham cubes. Top with reserved grated cheese.
Cover casserole with aluminum foil; seal dish in jumbo freezer bag. Freeze.
Place frozen casserole (covered with aluminum foil) in preheated 375ºF oven. Bake for 25 minutes; remove aluminum foil. Bake 25 minutes more, or until hot and bubbly.