Heat the oven to 375°F. Lightly grease 8 cups of a standard muffin pan.
In a large bowl, combine the ground beef, breadcrumbs, onion, egg, 2 tablespoons milk, 2 tablespoons ketchup, Worcestershire sauce, garlic powder, salt, and black pepper. Mix gently with your hands or a fork just until combined.
Divide the mixture among the 8 prepared muffin cups. Press it in lightly, leaving the tops rounded rather than packing the meat tightly. Brush each portion with the remaining 1 tablespoon ketchup.
Bake until the meatloaf muffins are cooked through and their centers reach 160°F, about 20 to 25 minutes. Let them rest in the pan for a few 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 very tender, about 15 to 20 minutes. Drain them well.
Mash the potatoes with the butter, warm milk, and 1/4 teaspoon salt until smooth and creamy. For a neat frosting effect, spoon the mash into a piping bag or a sturdy bag with a corner snipped off.
Pipe the mashed potatoes over the meatloaf muffins and serve warm.