Instructions

1. Pulse bread and 3 tablespoons chilled butter in a food processor to coarse crumbs, about 10 pulses and set aside.
2. Bring 4 quarts of water to a boil in a large pot. Add macaroni and 1 tablespoon salt and cook stirring often, until tender, just past al dente stage and drain.
3. Adjust your oven rack to the lower-middle position and heat broiler.
4. Melt remaining 5 tablespoons of butter in the now empty pot over medium-high heat. Add flour, mustard, 1 teaspoon salt and cayenne pepper and cook whisking constantly until the mixture becomes fragrant and deepens in color, about 1 minute. Gradually whisk in milk and bring the mixture to a boil, whisking occasionally, until thickened, about 5 minutes. Turn off the heat and slowly whisk in Monterey Jack and cheddar cheese until completely melted. Add macaroni to the sauce and cook over medium-low heat, stirring constantly until the mixture is heated through about 6 minutes.
5. Transfer the mixture to a 13x9 inch broiler safe baking dish and sprinkle the bread crumb mixture on top. Place under the broiler for 3 minutes, rotate baking dish and broil for another 3 minutes. Once the topping is a deep golden brown, remove from the oven and let cool for 5 minutes before serving.