In a large mixing bowl, combine ground beef, breadcrumbs, egg, milk, finely chopped onion, garlic powder, and barbecue sauce. Mix gently until just combined.

Shape the meat mixture into a loaf and place it into a greased 9x13-inch baking dish. Bake in the preheated oven for 25 minutes, or until it's nearly cooked through.

In a medium saucepan, melt butter over medium heat and whisk in flour to create a roux. Gradually whisk in milk, cooking until thickened.

Cook elbow macaroni according to package instructions in a large pot of salted water. Drain and return to the pot, then combine with cheese sauce.

Spread the creamy mac and cheese over the top of the meatloaf layer in the baking dish. Return the casserole to the oven and bake for an additional 20 minutes.

Remove the casserole from the oven and let it cool for about 10 minutes before slicing.