Cook the macaroni in a large pot of salted boiling water according to the package directions until just tender. Drain it, then return it to the pot.

Stir the butter, milk, and 3/4 cup of the cheddar into the hot macaroni until the cheese melts and coats the pasta. Set the mac and cheese aside to cool while you prepare the meatloaf.

In a large mixing bowl, combine the ground beef, breadcrumbs, egg, 2 tablespoons of the ketchup, mustard, garlic powder, smoked paprika, salt, and black pepper. Mix gently just until combined.

Lay a large sheet of foil on a work surface. Shape the beef mixture into a rectangle about 1 inch thick. Spoon the cooled mac and cheese down the center, leaving space around the edges. Bring the long sides of the beef mixture up and over the filling, then pinch the seams and ends closed to seal in the pasta.

Place the meatloaf seam-side down on a smoker-safe rack or a foil-lined tray. Arrange the bacon slices over the top, slightly overlapping them. Brush the bacon with the remaining ketchup.

Heat the smoker to 250°F. Smoke the meatloaf until the center of the beef reaches 160°F on an instant-read thermometer, usually about 2 to 2 1/2 hours. Sprinkle the remaining cheddar over the bacon during the last 10 minutes so it melts.

Rest the meatloaf for 10 minutes before slicing. Cut gently with a sharp knife to keep the mac and cheese filling in place.