Baked Macaroni Cheese

For Bread Crumb Topping:

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

For Macaroni and Cheese:

Bring 4 quarts salted water to a boil in a Dutch oven over high heat. Add macaroni; cook according to package instructions until pasta is tender (al dente). Drain pasta and set aside in colander.

Adjust oven rack to lower-middle position and heat broiler. Coat a broiler-safe 9 x 13-inch baking dish or pan with nonstick cooking spray or butter.

In the 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. Remove pan from heat source, whisk in cheeses and 1 teaspoon salt, stirring until cheeses are fully melted. Add pasta and cook over medium-low heat, stirring constantly, until mixture is steaming and heated through, about 4 to 6 minutes.