- Preheat an oven to 375°F. Butter a large rectangular baking dish.
- In a saucepan over medium heat, melt the 4 tablespoons butter. Add the flour and cook, stirring frequently, about 3 minutes. Whisk in the milk and cook, stirring frequently, until thickened, about 5 minutes. Whisk in the tomato paste.
- Add 1 1/2 cups of each cheese and stir until melted. Season with salt, black pepper and cayenne.
- Tear the bread into crumbs. In a small bowl, stir together the bread crumbs and the 2 tablespoons melted butter and season with salt.
- In a large bowl, combine the pasta and the cheese sauce, then pour into the prepared baking dish. Top with the remaining cheeses and the buttered bread crumbs.
- Bake until the cheeses are golden and bubbly, about 40 minutes. If the top becomes too dark, cover with aluminum foil. Let stand for 10 minutes before serving.