Break the macaroni into pieces about l 1/2 in long, put them into rapidly-boiling salted water and boil for about 20 min., or until the macaroni is tender.
(If not required for immediate use, cover the macaroni with cold water to prevent the pieces sticking together.) Cover the bottom of a well-buttered baking-dish with white sauce, sprinkle liberally with cheese, seasoning to taste, and add a layer of macaroni.
Repeat the layers, cover the last layer of macaroni thickly with sauce, sprinkle the surface lightly with breadcrumbs and add a few small pieces of butter.
Bake in a hot oven (425° F., Gas 7) for about 20 min., then serve in the dish in which it is cooked.