Preheat the oven to 350°F (175°C).
Cook the elbow macaroni according to package instructions until al dente. Drain and set aside.
In a large saucepan, melt 4 tablespoons of butter over medium heat.
Whisk in the all-purpose flour and cook, whisking constantly, for about 1-2 minutes until lightly golden and smooth.
Gradually whisk in the whole milk, continuing to whisk until the mixture thickens and starts to bubble, about 5-7 minutes.
Remove the sauce from heat and stir in the shredded cheddar cheese and mozzarella cheese until fully melted and smooth.
Season the cheese sauce with salt, black pepper, paprika, garlic powder, and onion powder. Stir well.
In a large mixing bowl, combine the cooked macaroni, shredded chicken, and cheese sauce. Mix until evenly combined.
Transfer the mixture to a greased 9×13 inch baking dish and spread evenly.
In a small bowl, combine the panko breadcrumbs with the melted butter and mix until the crumbs are evenly coated.
Sprinkle the breadcrumb mixture evenly over the top of the casserole.
Bake in the preheated oven for 25-30 minutes or until the top is golden brown and the casserole is bubbly.
Remove from the oven and let cool for 5 minutes before serving.