Baked Mac and Cheese

Steps

1. Preheat the oven to 375°F (190°C), conventional bake, rack in the middle. Butter a 9 x 13 inch (23 x 33 cm) baking dish with 2 teaspoons (10 g) of the butter. Grate the cheddar and Gruyère yourself, toss them together and set aside 2 oz (60 g) of the mix for the top.
2. Bring a large pot of water to a boil with the kosher salt. Cook the macaroni 2 minutes less than the package directions: it should still be firm at the center. Drain without rinsing and leave it in the colander.
3. While the pasta cooks, warm the milk in a saucepan or the microwave without letting it boil.
4. In a large heavy-bottomed saucepan, melt 3 tablespoons (40 g) of the butter over medium heat. When the foam subsides, add the flour all at once and whisk for 2 minutes: the roux should smell like a biscuit and stay pale, not brown.
5. Pour in the warm milk in three additions, whisking hard after each one to prevent lumps. Simmer for 6 to 8 minutes, scraping the bottom with the whisk, until the sauce coats the back of a spoon and a finger swiped through it leaves a clean line.
6. Take the pan off the heat. Add the mustard, half of the paprika, the garlic powder, salt and pepper, then stir in the grated cheese by the handful, letting each handful melt before adding the next. The sauce should be smooth and glossy.
7. Add the pasta to the sauce and stir well. It should look a little too loose: the pasta will soak up sauce in the oven. Transfer to the buttered dish and spread in an even layer.
8. Melt the remaining 1 1/2 tablespoons (20 g) butter and toss it with the panko and the rest of the paprika. Scatter the reserved cheese over the dish, then the crumbs in a thin, even layer.
9. Bake for 20 to 25 minutes, until the sauce bubbles at the edges and the top is golden and crisp. If the crust is still pale, run it under the broiler for 1 to 2 minutes, watching closely.
10. Let rest for 10 minutes out of the oven so the sauce firms up just enough to hold on a spoon. Sprinkle with snipped chives and serve.