In a large bowl, combine the shredded chicken, cream of chicken soup, sour cream, milk, cheddar cheese, salt, and pepper. Mix well.
Gently stir in the thawed hash brown potatoes until evenly coated.
Pour the mixture into a greased 9×13 inch baking dish.
In a small bowl, combine the crushed cornflakes and melted butter. Sprinkle this mixture evenly over the top of the casserole.
Bake for 40-50 minutes, or until the casserole is bubbly and the topping is golden brown.
Let the casserole stand for 5-10 minutes before serving.
Notes
For make-ahead convenience, assemble the casserole (without the topping) up to 24 hours in advance. Cover and refrigerate. Add the cornflake topping just before baking and increase the baking time by 5-10 minutes.
To transport to a potluck, let the casserole cool slightly after baking, then cover tightly with foil. You can reheat it gently in a 300°F (150°C) oven for about 15-20 minutes.
You can substitute cream of mushroom soup for cream of chicken soup if desired.
Add extra shredded cheese on top before the cornflake mixture for an even cheesier casserole.