Make the macaroni and cheese: Bring a large pot or Dutch oven of heavily salted water to a boil over high heat. Add 1 pound dried macaroni and cook until 1 minute less than al dente. Drain, rinse with cold water until cool to the touch, and set aside to drain thoroughly.

In the now-empty pot, heat 4 tablespoons of the unsalted butter over medium-high heat until foaming. Whisk in ¼ cup all-purpose flour, 1 ¾ teaspoons kosher salt, 1 ½ teaspoons powdered ground mustard, and and ¼ teaspoon cayenne if using until combined. Continue whisking until the mixture becomes fragrant and deepens in color, about 1 minute. Gradually whisk in 5 cups whole milk and bring to a simmer, whisking frequently.

Reduce the heat to medium and simmer, whisking frequently, until thickened slightly, about 2 minutes. Remove the pot from the heat. Whisk in 8 ounces shredded sharp cheddar cheese and 8 ounces shredded Monterey Jack cheese cheeses until smooth and fully melted. Set aside until cooled to just warm to the touch, about 30 minutes.

Meanwhile, melt the remaining 1 tablespoon unsalted butter and place in a medium bowl. Stir in 1 cup crushed cracker crumbs until combined.

When the sauce is cooled, add the pasta and stir to combine. Transfer to a 9x13-inch baking dish and spread into an even layer. Sprinkle the cracker crumbs evenly over the macaroni and cheese. Cover tightly with aluminum foil and refrigerate for up to 2 days.

When ready to bake: Heat the oven to 400°F. While the oven is heating, let the baking dish sit out at room temperature.

Bake covered until heated through and bubbling around the edges, about 45 minutes. Uncover and bake until the top is golden-brown, about 15 minutes more. Let sit 5 minutes before serving.