- Heat the oven to 375°F (190°C) and butter a 9 x 13-inch baking dish.
- Boil the macaroni in salted water for 2 minutes less than the package says. Drain.
- Melt the butter in a large saucepan over medium heat. Whisk in the flour and cook 2 minutes.
- Gradually whisk in the warm milk. Simmer, whisking often, until the sauce coats a spoon, 5 to 7 minutes.
- Take the pan off the heat. Stir in the mustard, paprika, cayenne and salt, then 2½ cups of the cheddar and all the Gruyère until melted.
- Fold in the macaroni and spread it in the dish. Scatter the remaining cheddar on top, toss the panko with the melted butter and sprinkle it over.
- Bake until bubbling and golden, about 25 minutes. Let it rest 5 minutes before serving.