Preheat oven to 375°F (190°C). Grease a muffin tin.
In a bowl, combine beef, breadcrumbs, ketchup, milk, onion, egg, Worcestershire, garlic powder, salt, and pepper. Mix gently.
Spoon mixture into muffin cups and press lightly to flatten tops.
Bake 20–25 minutes until browned and 160°F (71°C) inside. Let rest.
While baking, boil cubed potatoes in salted water for 15–20 minutes.
Drain and mash with butter, milk, salt, and pepper until smooth.
Top each muffin with mashed potatoes. Garnish with chives or parsley.