01 - Preheat the oven to 375°F. Grease a 9 x 13-inch baking dish.
02 - Bring a large pot of salted water to a boil. Add elbow macaroni and cook for 5 minutes. Add broccoli florets and cook for an additional 2 to 3 minutes, until the pasta is al dente and broccoli is just tender. Drain thoroughly.
03 - In a large saucepan, melt 4 tablespoons of unsalted butter over medium heat. Stir in all-purpose flour and cook, whisking constantly, for 1 to 2 minutes.
04 - Gradually whisk in whole milk and heavy cream. Continue stirring until the mixture thickens and begins to bubble, approximately 4 to 5 minutes.
05 - Remove from heat. Stir in shredded cheddar, mozzarella, grated Parmesan, Dijon mustard, garlic powder, onion powder, salt, and black pepper until the cheeses melt and sauce is smooth.
06 - Gently toss the drained macaroni and broccoli with the cheese sauce until evenly coated. Transfer the mixture into the prepared baking dish.
07 - In a small bowl, combine panko breadcrumbs, melted butter, and grated Parmesan cheese. Sprinkle evenly over the macaroni mixture.
08 - Bake for 20 to 25 minutes, until the topping is golden and sauce is bubbling. Optionally garnish with chopped fresh parsley. Allow to rest for 5 minutes before serving.