01 - Preheat oven to 350°F. Butter a 2-quart baking dish thoroughly.
02 - Bring a large pot of salted water to a rolling boil. Add macaroni and cook until just al dente, approximately 1-2 minutes less than package directions. Drain well 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 mixture is lightly golden and fragrant.
04 - Gradually pour in milk while whisking continuously to prevent lumps. Continue whisking until sauce thickens, approximately 3-5 minutes. Remove from heat.
05 - Stir in shredded 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 gently until every piece is 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 breadcrumbs are evenly coated with butter.
08 - Sprinkle breadcrumb mixture evenly over the macaroni surface. 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 - Remove from oven and let stand for 5 minutes to set. Garnish with chopped fresh parsley if desired before serving warm.