Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a 9x13-inch baking dish.
Stir together hash brown potatoes, chicken, cheddar cheese, condensed soup, sour cream, ½ cup melted butter, onion, garlic powder, salt, and black pepper in a large bowl until well combined. Spread mixture evenly in the prepared baking dish.
Combine crushed corn flakes and crushed potato chips in a bowl; sprinkle evenly over the casserole. Drizzle with 3 tablespoons melted butter.
Bake in the preheated oven until heated through and the topping is golden brown, 35 to 40 minutes. Serve warm.
