BOIL 3 quarts salt-free water in a 4-quart pot; ADD 2 cups uncooked elbow macaroni; SCRAPE bottom immediately with a wire whisk to avoid sticking; BOIL macaroni for 8-10 minutes.
POUR macaroni into colander in sink (remove all pasta from bottom of pot); RINSE macaroni and pot with cold water; ALLOW to drain.
RETURN empty pot to a new burner stovetop over medium-low heat; ADD 2 tablespoons margarine and allow to melt, then add 1 1/3 cups skim milk (or 50% milk with 50% water), 1/8 teaspoon white pepper, 1 pinch cayenne pepper, and 1 pinch garlic powder.
RAISE heat to medium until heated; ADD the contents of one (8 ounce) jar Cheez-Whiz (or half the contents of one 15-ounce jar), scraping all contents from jar using a flexible spatula; ADD 8 ounces shredded medium cheddar cheese.
WHISK mixture often without letting mixture boil, until thickened and glossy in appearance.
ADD rinsed drained macaroni; FOLD mixture together until well-blended.
POUR mixture into greased 8 x 8 baking dish.
BAKE at 300°F for 15 minutes.
REMOVE from oven with oven mitts; SPRINKLE top with 1/3 cup shredded medium cheddar cheese; ALLOW to set just long enough for the cheese to melt.