Spray a 9x13-inch baking dish with nonstick cooking spray. Set aside.

Bring a large pot of water to a boil and add the salt.

Using a cheese grater or food processor fitted with a grating attachment, grate the Cheddar and Swiss cheeses. Set aside.

Cut the cream cheese into 1/2-inch cubes. Set aside.

Add the elbow macaroni to the water. Cook, stirring occasionally, until the pasta is al dente, about 9 minutes. Drain the pasta in a large colander, and rinse thoroughly with cold water.

Preheat the oven to 375°F.

In a large pot over medium heat, melt the butter.

Once melted, add the flour. Whisk to thoroughly combine. Continue whisking until the roux turns slightly golden and begins to smell toasty, 1-2 minutes.

Slowly stream the milk into the pot while continuing to whisk. Cook the mixture, whisking steadily, until it begins to thicken and bubble, 4-5 minutes.

Once bubbling, turn off the heat. Add the mustard powder, salt, and black pepper.

Add the cheeses to the pot one handful at a time, whisking slowly after each addition.

Once the sauce is smooth, add the drained pasta noodles to the pot. Stir until combined.

Pour the pasta into the prepared baking dish.

Place the crackers in a plastic bag and crush them to the consistency of coarse sand using a rolling pin. Transfer the cracker crumbs to a large bowl.

Place the butter in a microwave-safe bowl. Microwave on high until butter is melted, about 30 seconds. Pour the melted butter over the cracker crumbs. Mix with a fork until the crumbs are evenly moistened.

Sprinkle the cracker crumbs in an even layer over the pasta.

Bake the mac and cheese on middle rack of oven until bubbling at the edges and the cracker crumbs are slightly browned, 25-28 minutes.

Check to see that mac and cheese is done. Remove from oven or add time as needed.

Allow macaroni and cheese to rest 15-20 minutes before serving.