Preheat oven to 375°F. Spray or grease a casserole dish or a 9x13 pan.
Cook macaroni as directed on the package in salted water until tender, drain.
While macaroni is cooking, combine 2 and ½ cups of cheese with flour, salt, pepper, and ground mustard.
In a large bowl, combine cheese and flour mixture with hot macaroni and stir to combine. Pour into the greased pan. Pour milk over macaroni. Top with remaining cheese. Dot with butter. Cover with aluminum foil.
Bake for 45 minutes (taking the foil off after 30 minutes if desired) or until firm and golden brown. You want to make sure there isn’t a lot of runny milk in it still, poke a knife in the middle if necessary. Some will soak in as it cools but it should be mostly absorbed.
(Optional) Let sit for 10 minutes before serving to further firm up.