Heat the oven to 375°F. Lightly grease the cups of a 12-cup muffin tin.
In a large bowl, combine the ground beef, egg, breadcrumbs, milk, chopped onion, Worcestershire sauce, garlic powder, salt, black pepper, and 2 tablespoons of the ketchup. Mix gently until just combined.
Divide the mixture among 8 muffin cups, filling each about three-quarters full. Press lightly to shape the tops, then spread the remaining ketchup over them.
Bake for 20 to 25 minutes, until the centers reach 160°F. Let the meatloaf muffins rest in the tin for 5 minutes, then lift them out and drain away any excess juices.
Stir the softened butter into the mashed potatoes. Spoon or pipe the potatoes onto the meatloaf muffins just before serving.