NOTE: This recipe can be prepared in two methods -- "crusty" (baked), or "creamy" (stovetop). SET oven rack to center position; PREHEAT oven to 375°F if using the "crusty" baked macaroni and cheese option in Step 9; SPRAY a 9 x 13 baking dish with non-stick baking spray; SHRED 16 ounces (1 lb.) sharp cheddar cheese; SET aside.
BRING 4 quarts (16 cups) water to a rolling boil; ADD 1 tablespoon salt and 4 cups dry medium elbow macaroni; BLANCH macaroni until just tender; DRAIN cooked macaroni in colander, scraping all pasta from pot.
RINSE drained macaroni in colander under cold running water for 30 seconds; ALLOW macaroni to drain; RETURN empty pot to stovetop on medium-low to medium heat.
MELT 6 tablespoons margarine in pot; ADD 2 tablespoons flour and 2 tablespoons cornstarch to make a roux; COOK over low heat, whisking until light golden (do not scorch).
POUR in 4 1/2 cups skim milk while whisking; ADD 1/2 teaspoon salt and 1/8 teaspoon citric acid; STIR; RAISE heat to medium.
ADD one (8 ounce) can Kraft Sharp Cheddar Easy Cheese; REDUCE heat to low; ADD 16 ounces shredded medium cheddar cheese; STIR constantly over lowest heat until mixture melts and becomes very smooth and consistent in texture.
ADD rinsed drained macaroni to pot; WHISK to separate any clumped macaroni and combine; ADD salt to taste if necessary.
POUR then scrape mixture into greased 9 x 13 baking dish (for make-ahead freezing only, use greased cake or loaf pans); SET baking dish over a large baking sheet covered with foil.
FOR CRUSTY MACARONI AND CHEESE: Leave baking dish uncovered; Bake at 375°F for 30 minutes. Allow to cool and set for 10 minutes. Serve and enjoy!
FOR CREAMY MACARONI AND CHEESE: Simmer over medium-low heat, stirring often until desired consistency is achieved. Serve and enjoy!