Baked Macaroni and Cheese

Method

1. Heat the oven to 375°F and butter a 9x13-inch baking dish. Bring a large pot of well-salted water to a boil.
2. Cook the macaroni 2 minutes short of the package's al dente time — it should still be chalky at the center. It finishes cooking in the sauce and in the oven, and fully cooked pasta turns to paste by the time the top browns. Drain but do not rinse.
3. Melt 4 tablespoons of the butter in the empty pot over medium. Whisk in the flour and cook 2 minutes, whisking constantly, until it smells faintly of toasted bread and looks like loose wet sand. Do not let it brown.
4. Pour in the warm milk in three additions, whisking each one completely smooth before the next. Cold milk hitting a hot roux seizes into lumps you cannot whisk out.
5. Simmer 5 to 7 minutes, whisking often, until the béchamel thickly coats a spoon and a finger drawn across leaves a clean line. Whisk in the dry mustard, nutmeg, cayenne and salt.
6. Pull the pot off the heat completely and add the cheddar and gruyere in three handfuls, stirring until each melts before adding the next. Above about 170°F the cheese proteins tighten and squeeze out fat, and the sauce breaks into a greasy, grainy mess.
7. Fold in the drained pasta. The mixture should look distinctly too saucy — the pasta drinks a surprising amount in the oven. Scrape it into the baking dish and smooth the top.
8. Melt the remaining 2 tablespoons butter and toss with the panko and parmesan until every crumb is coated, then scatter evenly over the top.
9. Bake 25 to 30 minutes, until the crumbs are deep golden and the sauce bubbles up thickly at the edges. Rest 10 minutes before serving so the sauce sets enough to hold a spoonful.