01 - Preheat oven to 350°F. Generously butter a 2-quart baking dish.
02 - Bring a large pot of salted water to a boil. Cook macaroni until al dente, about 1–2 minutes less than package directions. Drain thoroughly and set aside.
03 - In a large saucepan over medium heat, melt 3 tbsp butter. Whisk in flour and cook for 1–2 minutes, stirring constantly, until mixture is lightly golden and fragrant.
04 - Gradually pour in milk while whisking continuously to prevent lumps. Continue whisking until sauce thickens and bubbles, about 3–5 minutes.
05 - Remove from heat. Stir in cheddar, Gruyère, and mozzarella until completely melted and smooth. Add Dijon mustard, garlic powder, onion powder, paprika, salt, and pepper. Taste and adjust seasoning as needed.
06 - Add drained macaroni to the cheese sauce, stirring until evenly coated. Transfer mixture to the prepared baking dish, spreading evenly.
07 - In a small bowl, combine panko breadcrumbs, melted butter, and grated parmesan. Mix until butter is evenly distributed.
08 - Sprinkle breadcrumb mixture evenly over the macaroni. Bake uncovered for 20–25 minutes until golden brown and bubbling around edges. For extra crispiness, broil for 2–3 minutes at the end, watching closely to prevent burning.
09 - Let baked macaroni rest for 5 minutes before serving to allow sauce to set. Garnish with chopped fresh parsley if desired.