Classic Macaroni and Cheese

6 slices of white sandwich bread (good-quality, about 6 ounces), torn into rough pieces 3 tablespoons unsalted butter (cold), cut into 6 pieces For the Pasta and Cheese: 1 pound elbow macaroni 1 tablespoon table salt 5 tablespoons unsalted butter 6 tablespoons all-purpose flour 1 1/2 teaspoons mustard powder 1/4 teaspoon cayenne pepper (optional) 5 cups milk (see note) 8 ounces monterey jack cheese shredded (2 cups) 8 ounces sharp cheddar cheese shredded (2 cups) 1 teaspoon table salt

DIRECTIONS 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. 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. Drain pasta and set aside in colander. 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. 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.