Preheat the oven to 375°F and lightly grease a 12-cup muffin tin.
In a mixing bowl, combine the ground beef, breadcrumbs, egg, chopped onion, ketchup, garlic powder, salt, and black pepper. Mix until evenly blended but do not overwork the meat.
Divide the mixture evenly among 8 muffin cups, pressing it down gently to fill each cup. Bake for 25 minutes until the internal temperature reaches 160°F.
While the muffins bake, place the cubed potatoes in a saucepan and cover with water. Bring to a boil and cook for 15 minutes until tender.
Drain the potatoes, then return them to the saucepan. Add the milk and butter. Mash until smooth.
Remove the muffins from the oven and let them rest for 5 minutes. Pipe or spoon the mashed potatoes over each muffin to create the frosting effect before serving.