Preheat the oven to 350°F (175°C).
Cook the elbow macaroni in a large pot of salted boiling water according to package instructions until al dente, about 7-8 minutes. Drain and set aside.
In a large saucepan, melt 4 tablespoons of butter over medium heat.
Whisk in the all-purpose flour and cook for 1-2 minutes, stirring constantly, until the mixture is lightly golden and bubbly.
Gradually whisk in the warmed whole milk, continuing to whisk until the sauce thickens and is smooth, about 5-7 minutes.
Remove the sauce from heat and stir in the shredded sharp cheddar cheese and shredded mozzarella cheese until fully melted and smooth.
Season the cheese sauce with salt, black pepper, garlic powder, onion powder, and smoked paprika. Stir to combine.
Add the cooked macaroni and shredded chicken to the cheese sauce and stir until everything is evenly coated.
Transfer the mac and cheese mixture into a greased 9×13 inch baking dish and spread it out evenly.
In a small bowl, mix the plain breadcrumbs with the melted 2 tablespoons of butter until the crumbs are evenly coated.
Sprinkle the buttered breadcrumbs evenly over the top of the mac and cheese mixture.
Bake in the preheated oven for 25-30 minutes, or until the casserole is bubbly and the breadcrumb topping is golden brown.
Remove from the oven and let the casserole rest for 5 minutes before serving.