PUT torn bread and butter bits In a food processor (or similar device); PULSE until crumbs are no larger than 1/8", about 12 one-second pulses; SET aside.

In a large sauce pan over high heat, bring 2 quarts water to a BOIL.

ADD macaroni and salt; COOK until pasta is tender (just past "al dente"--in fact it is better to overcook than undercook).

DRAIN pasta and set aside in colander.

Using the same sauce pan over medium-high heat, HEAT butter until foaming.

ADD flour, mustard, cayenne; WHISK well to combine.

Continue WHISKING until mixture becomes fragrant and deepens in color, about 1 minute.

Gradually WHISK in milk; bring mixture to a BOIL, WHISKING constantly (mixture must reach full boil to fully thicken).

REDUCE heat to medium; SIMMER, WHISKING occasionally, until thickened to the consistency of heavy cream, about 5 minutes.

REMOVE from heat; ADD grated cheeses and salt; continue WHISKING until cheeses are fully melted.

ADJUST oven broiler rack to lower-middle position; start to PREHEAT at broil setting.

In the meantime, RETURN cheese mixture to stove; ADD pasta; COOK over medium-low heat, STIRRING constantly, until mixture is steaming and heated through, about 6 minutes.

TRANSFER mixture to a baking dish (use an 8 x 8 dish for serving 4); SPRINKLE evenly with bread crumb mixture.

BROIL until crumbs are a deep golden brown, 3 to 5 minutes, ROTATING dish if necessary for even browning.

ALLOW to cool 5 minutes before serving.