Preheat the oven to 350 degrees . In a large pot of cold, salted water, bring the potatoes to a boil over high heat. Lower the heat to medium and cook until tender, about 20 minutes; drain. When cool enough to handle, remove and discard the skins and transfer the potatoes to a bowl. Mash with the warm milk and 2 tbsp. butter until almost smooth. Season with salt and pepper; let cool to room temperature.
Meanwhile, in a large bowl, combine the beef, eggs, worcestershire sauce and onion powder (if using) with two-thirds of the scallions, two-thirds of the parsley, 4 tbsp. of the sour cream and 1/2 cup of the mashed potatoes; season.
Line a baking sheet with parchment paper. Transfer the beef mixture to the pan and shape into an 8-by-4-inch loaf. Bake for 30 minutes. Stir the remaining scallions, parsley and sour cream into the remaining mashed potatoes. Pat potatoes onto the meatloaf and dot with the remaining 1 tbsp. butter. Bake 15 minutes longer. Let meatloaf rest for 5 minutes before serving.