Allow macaroni and cheese to rest 15-20 minutes before serving.For this recipe, you need a cast iron pan because it can be easily transferred into the oven.Check to see that mac and cheese is done.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.Sprinkle the cracker crumbs in an even layer over the pasta.Mix with a fork until the crumbs are evenly moistened. Pour the melted butter over the cracker crumbs. Microwave on high until butter is melted, about 30 seconds. Place the butter in a microwave-safe bowl.Transfer the cracker crumbs to a large bowl. Place the crackers in a plastic bag and crush them to the consistency of coarse sand using a rolling pin.Pour the pasta into the prepared baking dish.Once the sauce is smooth, add the drained pasta noodles to the pot.Add the cheeses to the pot one handful at a time, whisking slowly after each addition.Add the mustard powder, salt, and black pepper. Cook the mixture, whisking steadily, until it begins to thicken and bubble, 4-5 minutes.