Preheat the oven to 350°F. Grease a 9x13-inch casserole dish and set it aside.
In a large mixing bowl, combine the thawed hash browns, shredded cooked chicken, cream of chicken soup, cream of mushroom soup, shredded cheese, sour cream, milk, garlic salt, onion powder, and pepper.
Mix until well combined.
Spread the mixture evenly in the prepared casserole dish.
In a separate bowl, mix the crushed cornflakes and melted butter. Sprinkle this mixture over the top of the casserole.
Bake for 45-50 minutes or until the casserole is bubbly and the topping is golden brown.
Remove from the oven and let it cool for a few minutes before serving.