Peel and quarter the potatoes, then place them in a large pot and cover with cold water. Bring to a boil and cook for 15–20 minutes, or until fork tender.
In a large bowl, combine the ground beef, breadcrumbs, Parmesan cheese, egg, salt, pepper, garlic powder, parsley, and cowboy butter seasoning. Mix until just combined, then roll into evenly sized meatballs.
Heat a large skillet over medium-high heat. Sear the meatballs, turning occasionally, until browned on all sides and cooked through, about 10–12 minutes. Remove from the skillet and set aside.
In the same skillet, melt the butter. Stir in the heavy cream, Worcestershire sauce, Dijon mustard, garlic powder, parsley, and cowboy butter seasoning. Simmer for 2–3 minutes until slightly thickened.
Return the meatballs to the skillet and spoon the sauce over them. Simmer for another 2–3 minutes until heated through.
Drain the cooked potatoes and return them to the pot. Add the butter, milk, heavy cream, salt, and pepper. Mash until smooth and creamy. Add a little more milk or cream if needed to reach your desired consistency.
Serve the cowboy butter meatballs over a generous scoop of mashed potatoes and spoon the extra sauce over the top. Garnish with additional parsley and Parmesan cheese, if desired.
