Prep and heat the oven (20 minutes): position a rack in the center of the oven and heat the oven to 350°F (175°C). Butter a 9×13-inch baking dish with the 1 teaspoon of unsalted butter, or coat it with cooking spray. Shred all the cheese by hand while it is still cold from the fridge — the 12 ounces of sharp cheddar (about 3 cups) and the 8 ounces of Gruyère (about 2 cups) — and toss them together in one large bowl, then set aside 1 cup of the cheddar from that total for the top of the dish. Measure the flour, milk, cream and seasonings, and stir the topping together in a small bowl: the 1 1/2 cups of panko, the 2 tablespoons of melted butter, the 1 ounce of finely grated Parmesan (about 1/4 cup) and the 1/2 teaspoon of smoked paprika for the topping, mixed until the crumbs look like damp sand. Bring a large pot of water to a boil for the pasta. This 20 minutes of prep is what makes the sauce calm to cook — cheese sauce does not wait well, so everything needs to be ready before the butter melts.

Cook the pasta (7 minutes): add the 1 tablespoon of kosher salt and the 1 pound of elbow macaroni to the boiling water, stir well so nothing sticks, and cook for 7 minutes, until the pasta is just shy of al dente — tender at the edges but still firm with a definite bite in the center. The elbows will finish cooking in the oven, and pasta cooked all the way on the stove turns soft and swollen after baking. Drain the pasta well, return it to the warm empty pot off the heat, and set it aside — do not rinse it, because the light starch on its surface helps the sauce cling.

Make the cheese sauce (8 minutes): in a large, heavy pot or Dutch oven, melt the 6 tablespoons of unsalted butter over medium heat. Whisk in the 1/2 cup of flour and cook, whisking constantly, for 2 minutes, until the roux is smooth, bubbling and pale golden — this cooks out the raw flour taste without browning it. Slowly pour in the 4 cups of whole milk a little at a time, whisking hard after each addition so no lumps form, then whisk in the 1 cup of heavy cream. Cook for 4 minutes, whisking often and then constantly as it thickens, until the béchamel comes to a gentle simmer and coats the back of a spoon thickly. Whisk in the 1 teaspoon of dry mustard powder, the 1/2 teaspoon of garlic powder, the 1/2 teaspoon of smoked paprika for the sauce, the 1 teaspoon of fine sea salt, the 1/2 teaspoon of black pepper and the 1/8 teaspoon of cayenne pepper. Then take the pot completely off the heat and wait for 1 minute before adding the cheese, and use the final 1 minute of those 8 minutes to stir in the mixed cheeses in 3 handfuls, keeping the reserved 1 cup of cheddar aside, and stir gently after each handful until it melts completely before adding the next. Off the heat, the cheese melts smooth and glossy; in a boiling sauce, it can turn grainy and oily.

Combine and fill the dish: pour the warm, drained pasta into the cheese sauce and fold gently until every elbow is coated and the pot looks generously saucy — it should look a little looser than you expect, because the pasta keeps absorbing sauce in the oven. Scrape everything into the prepared 9×13-inch dish and spread it into an even layer. Scatter the reserved 1 cup of shredded cheddar evenly over the top, then sprinkle the buttered panko topping over the whole surface, edge to edge, so every serving gets some crunch.

Bake (25 minutes): bake, uncovered, at 350°F (175°C) for 25 minutes, until the sauce is bubbling up around the edges, the cheese on top has melted, and the panko crust is deep golden brown. If the top is browning much faster than the center is bubbling, lay a sheet of foil loosely over the dish for the remaining minutes — but in a 9×13-inch dish at 350°F (175°C), 25 minutes is normally exactly right for a crisp top and a creamy, scoopable center.

Rest and serve (5 minutes): remove the dish from the oven and let the mac and cheese rest, uncovered, for 5 minutes before serving. This short rest is part of the recipe, not an afterthought: the sauce thickens from runny to creamy and coats the pasta instead of pooling in the dish, and the servings hold together on the spoon. Serve warm, straight from the dish, spooning down through the golden crust so each of the 10 side servings (or 6 main servings) gets both crunch and cream.