NOTE: IF YOU DON'T HAVE SHARP CHEDDAR CHEESE, SUBSTITUTE I CUP OF THE CHEDDAR CHEESE WITH GRATED ROMANO CHEESE. MEASURE 4 LEVEL CUPS (ABOUT 14 OUNCES) OF DRY MACARONI (DON'T USE A FULL POUND). USE WHOLE MILK ONLY (STEP ONE PREP): Preheat oven to 350°F ; Lightly butter or spray a 2.8 - 3 quart casserole dish. Shred 20 ounces sharp cheddar cheese on a medium grate. Set aside approximately 1 cup of shredded cheese for the final garnish. Prepare the breadcrumb topping if using (melt butter, then toss all ingredients well). Set all aside until needed.
INTO a large pot with an accompanying tight-fitting lid, add 12 cups (3 quarts) water enough kosher salt to season moderately; BRING to a rapid boil.
ADD macaroni to boiling water and bring back to a rapid boil; COOK, stirring constantly, for 3 minutes; COVER pot and remove from heat; LET stand 10 minutes; DRAIN in a colander placed in sink; RINSE with hot water and allow to drain again.
IN the previous emptied pot over a low heat setting, add the butter in tablespoons until melted; SPRINKLE in the flour and stir; RAISE heat to medium low and cook until light golden, bubbly and slightly nutty in aroma, stirring as needed (do not let mixture turn brown or you will need to wash the pot and start over!).
RAISE heat to medium-high; GRADUALLY whisk in the milk; WHISK in the kosher salt, mustard powder, garlic powder, cayenne pepper and nutmeg; CONTINUE whisking until mixture begins to steam and bubble; REDUCE heat and cook 1-2 minutes until fully thickened; REMOVE from heat and let stand 1-2 minutes.
STIR in the sharp cheddar cheese (except the reserved 1 cup) until blended and creamy; FOLD in the macaroni and blend until creamy and macaroni is clump-free.
POUR mixture into prepared baking dish; SPRINKLE evenly with reserved 1 cup sharp cheddar cheese; TOP evenly with combined BUTTERED BREADCRUMB TOPPING ingredients (or if not using breadcrumb topping, sprinkle with 2 tablespoons powdered parmesan cheese).
SLIDE out oven rack using oven mitts; PLACE casserole dish in center of prepared baking sheet; SET oven timer for 35 minutes.
BAKE until timer sounds; CHECK to make sure breadcrumbs are golden brown and edges are bubbling (if not, return to oven and continue baking); REMOVE from oven and place on cooling rack; COOL and let stand a few minutes before serving.