INSTRUCTIONS
1. Adjust an oven rack to the middle position and heat the oven to 350 degrees. Melt 4 tablespoons of the butter. Pulse the bread, melted butter, and Parmesan in a food processor to coarse crumbs, about 6 pulses. Transfer to a bowl.
2. Bring 4 quarts water to a boil in a large pot. Add 1 tablespoon salt and the macaroni and cook until almost tender. Reserve ½ cup of the pasta cooking water. Drain the pasta with cold water until cool, then drain well.
3. Melt the remaining 4 tablespoons butter in the pot. Add the flour and cook over medium-high heat, stirring constantly, until light brown, about 1 minute. Slowly whisk in the evaporated milk, hot sauce, mustard, nutmeg, and 2 teaspoons salt and cook until slightly thickened, about 4 minutes.
4. Turn off the heat, whisk in the reserved pasta cooking water and cheeses until melted. Stir in the macaroni and transfer to a 13 by 9-inch baking dish. Top evenly with the bread-crumb mixture and bake until the cheese is bubbling around the edges and the top is golden brown, 20 to 25 minutes. Let sit for 5 to 10 minutes and serve.
