Ensure hash browns have thawed completely overnight in refrigerator. Preheat oven to 350 degrees F. Spray 9x13 inch baking dish with cooking spray.
In large mixing bowl, combine 1/2 cup melted butter, cream of chicken soup, sour cream, and cream cheese cubes. Mix until mostly smooth.
Stir in grated onion, garlic powder, salt, and pepper until well combined.
Add thawed hash browns and shredded cheddar cheese to bowl. Fold together gently until potatoes are evenly coated.
Transfer mixture to prepared baking dish and spread evenly.
Place cornflakes in large zip-top bag and crush lightly. Add 1/4 cup melted butter, seal, and shake until cornflakes are evenly coated.
Sprinkle buttered cornflake mixture evenly over top of casserole.
Bake uncovered for 40 to 45 minutes until edges are bubbling and topping is golden brown.
Let rest for 5 minutes before serving to allow filling to set.