Preheat oven to 375 degrees. Cook pasta 6 minutes. Drain.
In a medium saucepan, combine cornstarch, salt, dry mustard, and pepper. Stir in milk until smooth.
Add butter, stirring constantly. Bring to a boil over medium-high heat. Boil 1 minute. Remove from heat.
Stir in 1&3/4 cups cheese until melted. Add cooked pasta. Pour into greased 2-quart casserole dish.
Sprinkle with remaining 1/4 cup cheese. Bake uncovered 25 minutes, or until lightly browned.
Makes 6 servings.