Preheat the oven to 375°F (190°C). Grease the muffin tin with cooking spray or lightly oil it.
In a mixing bowl, combine the ground beef, bread crumbs, chopped onion, milk, egg, Worcestershire sauce, garlic powder, salt, and pepper. Mix well until all ingredients are fully incorporated.
Spoon the meat mixture evenly into the muffin tin, filling each cup about three-quarters full.
Bake in the preheated oven for 20-25 minutes, or until the meatloaf muffins are cooked through and the internal temperature reaches 160°F (70°C).
While the muffins are baking, prepare the mashed potatoes. Boil the chopped potatoes in a saucepan of salted water until fork-tender, approximately 15 minutes. Drain and return to the pot.
Add the butter, sour cream, and milk to the drained potatoes. Mash until smooth. Adjust seasoning if desired.
Once the muffins are done, let them cool for a few minutes, then carefully remove them from the muffin tin.
For decorative frosting, transfer the mashed potatoes to a piping bag (or use a spoon) and pipe or spread it on top of each mini meatloaf muffin.
Optionally, sprinkle with chives or paprika for added flavor and color.