01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish.
02 - Bring a large pot of salted water to a boil. Cook macaroni until just al dente, about 1–2 minutes less than package instructions. Drain and set aside.
03 - Melt butter in a large saucepan over medium heat. Whisk in flour and cook, stirring, for 1 to 2 minutes until pale golden.
04 - Gradually whisk in warmed milk and heavy cream. Stir constantly until mixture is smooth. Continue cooking 4 to 5 minutes, until thickened.
05 - Add garlic powder, mustard powder if using, salt, and black pepper. Remove from heat and fold in cheddar and mozzarella until cheeses are fully melted and sauce is smooth.
06 - Mix drained macaroni into cheese sauce, ensuring even coverage. Transfer mixture to prepared baking dish.
07 - In a small bowl, combine breadcrumbs with melted butter and Parmesan if desired. Evenly sprinkle mixture over the surface of the pasta.
08 - Bake for 25 to 30 minutes, until topping is golden and the dish is bubbling. Allow to rest for 5 minutes before serving.