01 - Preheat oven to 375°F. Grease a 9x13 inch baking dish with butter or cooking spray.
02 - Bring a large pot of salted water to a boil. Add macaroni and cook for 6 minutes. Add broccoli florets and continue cooking for 2 minutes until pasta is al dente and broccoli is bright green. Drain well in a colander.
03 - In a large saucepan, melt 4 tbsp butter over medium heat. Add flour and cook for 2 minutes, stirring constantly to form a smooth paste. Do not let it brown.
04 - Gradually whisk in milk and heavy cream. Continue whisking and cook for 5 minutes until sauce thickens and bubbles. Remove from heat, then stir in cheddar, Gruyère, Dijon mustard, garlic powder, onion powder, salt, and pepper until completely smooth and melted.
05 - Add the cooked macaroni and broccoli to the cheese sauce. Mix gently until evenly coated. Pour the mixture into the prepared baking dish and spread evenly.
06 - In a small bowl, combine panko breadcrumbs, grated Parmesan, and melted butter. Mix until breadcrumbs are evenly coated.
07 - Sprinkle the breadcrumb mixture evenly over the macaroni. Bake for 20-25 minutes until sauce is bubbling around edges and topping is golden brown.
08 - Remove from oven and let rest for 10 minutes before serving to allow sauce to set slightly for easier portioning.