Heat the oven to 375°F. Lightly grease 8 cups of a standard muffin pan.
Add the ground beef, breadcrumbs, egg, diced onion, Worcestershire sauce, garlic powder, salt, pepper, and 2 tablespoons of the barbecue sauce to a mixing bowl. Mix gently until combined, taking care not to compact the beef.
Divide the mixture among the prepared muffin cups and press it in lightly. Spoon the remaining barbecue sauce over the tops.
Bake for 20 to 25 minutes, until the meatloaf muffins are cooked through. Let them rest in the pan for 5 minutes, then carefully lift them out.
While the muffins bake, place the potatoes in a saucepan and cover with water. Bring to a boil, then simmer until the potatoes are fork-tender, about 15 to 20 minutes. Drain well.
Mash the potatoes with the butter and warmed milk until smooth. Stir in the cheddar until melted. Transfer the mashed potatoes to a piping bag or a sturdy resealable bag with one corner snipped off.
Pipe a cheddar mashed potato swirl onto each meatloaf muffin and serve warm.