Classic Macaroni and Cheese (America's Test Kitchen)

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 and 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 and set aside in colander.

In now empty Dutch oven, heat butter over medium-high heat until foaming.

Add flour, mustard, and cayenne 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" baking dish and sprinkle evenly with bread crumb mixture.

Broil until crumbs are deep golden brown, 3 to 5 minutes, rotating pan if necessary for even browning.

Cool about 5 minutes, then serve.