Preheat oven to 375°F. Grease a 9x9-inch baking dish or line with parchment paper.
In a large bowl, combine the ground beef, breadcrumbs, milk, egg, ketchup, garlic powder, onion powder, salt, and pepper. Mix gently until just combined.
Spread the meat mixture evenly in the bottom of the prepared baking dish. Bake uncovered for 20-25 minutes.
Warm the mashed potatoes and stir in sour cream and butter. Season with salt and pepper.
Carefully spread the mashed potatoes over the baked meatloaf layer.
Sprinkle shredded cheddar evenly over the top. Bake an additional 10-15 minutes until the cheese is melted and bubbly.
Let cool for 5-10 minutes. Garnish with chopped parsley if desired. Slice and serve.
Notes
Mix the meatloaf just until combined to avoid a dense texture. Leftover mashed potatoes work well for the topping. Can be assembled up to 24 hours ahead and refrigerated before baking. Store leftovers in the refrigerator up to 4 days, or freeze sliced portions up to 2 months.