8 tablespoons (1 stick) unsalted butter, plus more for the casserole dish
5 1/2 cups milk
1/2 cup all-purpose flour
2 teaspoons kosher salt
1/4 teaspoon freshly grated nutmeg
1/4 teaspoon freshly ground black pepper
1/4 teaspoon cayenne pepper
4 1/2 cups grated sharp white cheddar
2 cups grated Gruyere
1 pound elbow macaroni
1. Heat the oven to 375 degrees. Butter a 3-quart casserole dish; set aside. Place bread pieces in a medium bowl. Melt 2 tablespoons of butter. Pour melted butter in the bowl with the bread, and toss. Melt remaining 6 tablespoons of butter in a high sided pot over medium heat. When butter bubbles, add flour. Whisk for 1 minute.
2. Slowly pour milk into the flour-butter mixture while whisking. Continue cooking and whisking until mixture bubbles and becomes thick.
3. Remove pan from heat. Stir in salt, nutmeg, black pepper, cayenne pepper, 3 cups of cheddar and 1 1/2 cups of Gruyere. Set cheese sauce aside.
4. Add macaroni to a large pot of boiling water; cook 2-3 minutes fewer than the manufacturer's directions, until outside of pasta is cooked and inside is underdone. Drain macaroni and rinse under cold water. Drain well and then add macaroni to the cheese sauce.
5. Pour macaroni and cheese mixture into the casserole dish. Sprinkle remaining cheese on top, and then scatter breadcrumbs on top of that. Bake until browned on top. 20-30 minutes.
6. Eat and enjoy...possibly seconds, thirds, or fourths.