How to Make It
Heat oven to 400° F. Butter a 9-by-13-inch baking dish or some other shallow 3-quart baking dish.
Cook the pasta according to the package directions.
Wipe out the past pot and melt the butter over medium heat. Add the flour and cook, stirring constantly, for 2 minutes (do not let it darken). Still stirring, slowly add the milk. Cook, stirring occasionally, until the sauce slightly thickens, 6 to 8 minutes.
Add the Gruyère, Cheddar, cayenne, and 1½ teaspoon salt and cook just until the cheeses melt. Mix in the pasta. Transfer the pasta mixture to the prepared baking dish and bake until golden, 25 to 30 minutes.