Preheat the oven to 375°F (190°C) and lightly grease a 12-cup muffin pan.
In a large bowl, combine ground beef, ground pork, breadcrumbs, egg, Parmesan cheese, garlic, parsley, Italian seasoning, onion powder, salt, pepper, and milk. Mix until just combined.
Divide the meat mixture evenly among the muffin cups, pressing gently into each cup.
Bake for 20–25 minutes or until the internal temperature reaches 160°F (71°C). Drain any excess grease if needed.
Meanwhile, boil the potatoes in salted water for 15–18 minutes, until fork tender. Drain well.
Mash the potatoes with butter, warm milk, sour cream, salt, and pepper until smooth and fluffy.
Allow the baked meatballs to cool for about 5 minutes.
Transfer the mashed potatoes to a piping bag fitted with a large star tip and pipe decorative swirls onto each meatball cupcake. Alternatively, spoon the potatoes on top.
Sprinkle with shredded cheddar cheese, bacon bits, and chopped parsley.
Serve immediately while warm.