1. Preheat your oven to 375°F (190°C) and lightly grease a 12-cup muffin tin with cooking spray or oil.
2. In a large mixing bowl, combine the ground beef, egg, breadcrumbs, ketchup, milk, chopped onion, garlic powder, Worcestershire sauce, salt, and pepper. Mix until the ingredients are well incorporated, being careful not to overwork the meat.
3. Divide the meat mixture evenly among the muffin cups, pressing gently to shape. Top each meatloaf muffin with a small spoonful of ketchup.
4. Bake in the preheated oven for 20-25 minutes, or until the meatloaf muffins are cooked through and no longer pink in the center. An internal thermometer should read 160°F (71°C) when inserted into the center of a muffin.

5. Remove from the oven and let stand for 5 minutes before carefully removing from the muffin tin.
6. Serve warm, garnished with additional ketchup or barbecue sauce if desired. Enjoy your delightful Mini Meatloaf Muffins paired with your chosen sides for a comforting and satisfying meal.