1. For the bread crumbs: Pulse bread and butter in food processor until crumbs are no larger than 1/8 inch, ten to fifteen 1-second pulses. Set aside.

2. For the pasta and cheese: Adjust oven rack to lower-middle position and heat broiler. Bring 4 quarts water to boil in Dutch oven over high heat. Add macaroni and 1 tablespoon salt; cook until pasta is tender. It's crucial to cook the pasta until tende r- -just past the "al dente" stage. In fact, overcooking is better than undercooking. Drain and set aside in colander.

3. In now-empty Dutch oven, heat butter over medium-high heat until foaming. Add flour, mustard, and cayenne (if using) and whisk well to combine. Continue whisking until mixture becomes fragrant and deepens in color, about 1 minute. Gradually whisk in milk; bring mixture to boil, whisking constantly (mixture must reach full boil to fully thicken). Reduce heat to medium and simmer, whisking occasionally, until thickened to consistency of heavy cream, about 5 minutes. Off heat, whisk in cheeses and 1 teaspoon salt until cheeses are fully melted. Add pasta and cook over medium-low heat, stirring constantly, until mixture is steaming and heated through, about 6 minutes.

4. Transfer mixture to broiler-safe 9-by 13-inch baking dish and sprinkle evenly with bread crumbs. Broil until crumbs are deep golden brown, 3 to 5 minutes, rotating pan if necessary for even browning. Cool about 5 minutes, then serve.

NOTES:

The recipe can be halved and baked in an 8-inch-square, broilersafe baking dish.

If desired, offer celery salt or hot sauce (such as Tabasco) for sprinkling at the table.