01 - Preheat oven to 375°F (175°C). Lightly grease a 9x13-inch baking dish.
02 - Bring a large pot of salted water to a rolling boil. Cook elbow macaroni until just al dente, approximately 1 to 2 minutes less than package directions recommend. Drain thoroughly and set aside.
03 - Melt unsalted butter in a large saucepan over medium heat. Incorporate all-purpose flour and whisk continuously for 1 to 2 minutes until a pale golden paste forms.
04 - Gradually whisk in the warmed whole milk and heavy cream until the mixture is smooth. Continue to cook, stirring constantly, for 4 to 5 minutes until the sauce has thickened.
05 - Stir in garlic powder, mustard powder (if using), salt, and black pepper. Remove the saucepan from heat. Gently fold in the shredded cheddar and mozzarella cheeses until completely melted and the sauce is velvety smooth.
06 - Gently mix the drained macaroni into the prepared cheese sauce, ensuring even coating of all pasta strands. Transfer the mixture to the prepared baking dish.
07 - In a small bowl, combine the breadcrumbs with the melted butter. If desired, mix in the grated Parmesan cheese. Evenly sprinkle this topping mixture over the entire surface of the macaroni and cheese.
08 - Bake for 25 to 30 minutes, or until the topping is golden brown and the dish is bubbling around the edges. Allow the macaroni and cheese to rest for 5 minutes before serving to allow the sauce to set.