Step 1

For the bread crumbs, pulse bread and butter in food processor until crumbs and butter are well combined and crumbs are fine. Set aside.

Step 2

For the pasta and cheese, bring 4 quarts water to boil in a large pot or Dutch oven over high heat. Add macaroni and 1 tablespoon salt; cook until pasta is tender, about 1 minute past the al dente stage. Drain pasta and set aside in colander.

Step 3

Adjust a rack in the oven to middle or lower-middle position and preheat the broiler.

Step 4

In now-empty Dutch oven or pot, heat butter over medium 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.

Step 5

Off heat, whisk in cheeses, a handful at a time, 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.

Step 6

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. Watch carefully so the crumbs don't burn! Cool about 5 minutes. Serve.