Preheat oven to 425°F. Coat a 13- x 9-inch baking dish with cooking spray. Stir together hash browns, soup, sour cream, scallions, butter, chili powder, salt, pepper, and 1 1/2 cups of the cheese in a large bowl. Spoon mixture into prepared dish; sprinkle with remaining 1/2 cup cheese. Bake in preheated oven until potatoes are tender and cheese is melted and bubbly, 25 to 30 minutes. Sprinkle with parsley. Serve hot.