01 - Preheat oven to 350°F. Generously butter a 2-quart baking dish and set aside.
02 - Bring a large pot of salted water to a rolling boil. Cook macaroni until just al dente, approximately 1-2 minutes less than package directions. Drain thoroughly and set aside.
03 - In a large saucepan over medium heat, melt 3 tablespoons butter. Whisk in flour and cook for 1-2 minutes, stirring constantly until the mixture turns lightly golden and fragrant.
04 - Gradually pour in milk while whisking vigorously to prevent lumps. Continue cooking and whisking until the sauce thickens and coats the back of a spoon, about 3-5 minutes.
05 - Remove pan from heat. Stir in cheddar, Gruyère, and mozzarella until completely melted and silky smooth. Add Dijon mustard, garlic powder, onion powder, paprika, salt, and pepper. Taste and adjust seasonings as needed.
06 - Add drained macaroni to the cheese sauce, folding gently until every piece is evenly coated. Transfer the mixture to the prepared baking dish, spreading into an even layer.
07 - In a small bowl, combine panko breadcrumbs with melted butter and grated parmesan. Sprinkle the mixture evenly across the surface of the macaroni.
08 - Bake uncovered for 20-25 minutes until the top is deeply golden and the sauce is bubbling around the edges. For extra crispiness, broil for 2-3 minutes, watching carefully to prevent burning.
09 - Allow the dish to rest for 5 minutes before serving to let the sauce set. Garnish with fresh chopped parsley if desired.