Preheat oven to 350°F (175°C). Grease a 9x13-inch baking dish.
In a large bowl, combine sour cream, cream of chicken soup, melted butter, garlic powder, onion powder, salt, and pepper.
Stir in hash browns, chopped chicken, and shredded cheddar until well mixed.
Spread mixture evenly in the prepared baking dish.
In a small bowl, mix crushed cornflakes with 2 tablespoons of melted butter. Sprinkle evenly over the casserole.
Bake uncovered for 45-50 minutes, or until bubbly and golden on top.