Preheat the oven to 350 degrees Fahrenheit. Spray a 13 x 9 x 2-inch baking dish with nonstick cooking spray and set aside.
In a large bowl, add 1 (30-ounce) bag shredded hash brown potatoes, 3 cups cooked shredded chicken, 2 (10.5-ounce) cans cream of chicken soup, 2 cups sour cream, 1½ teaspoons onion powder, 1 teaspoon salt, ½ teaspoon ground pepper, and 2½ cups shredded sharp cheddar cheese. Stir to combine well.
Transfer the potato mixture to the prepared baking dish and spread it evenly.
In a medium bowl, add ¼ cup melted unsalted butter and 2 cups crushed cornflake cereal. Stir to combine, then sprinkle evenly over the potato mixture.
Bake for 45 minutes, or until hot and bubbly.
