Preheat your oven to 375°F (190°C). Lightly grease a standard 9x13 inch baking dish.

Ensure the frozen hashbrowns have thawed slightly (about 30 minutes at room temperature is ideal) to help them mix evenly. If the potatoes are still frozen solid, the casserole may take longer to bake.

In a large mixing bowl, whisk together the cream of chicken soup, sour cream, 1/2 cup of the melted butter (from the casserole base ingredients), garlic powder, onion powder, salt, and pepper until the mixture is smooth and well combined.

Add the slightly thawed hashbrowns, the cooked and shredded chicken, and 2 cups of the shredded cheddar cheese (reserve the remaining 1/2 cup for the final layer) to the bowl with the creamy sauce.

Use a sturdy spoon or spatula to gently fold all the ingredients together until the hashbrowns and chicken are fully coated in the creamy mixture. Pour the entire hashbrown and chicken mixture into the prepared 9x13 inch baking dish and spread it into an even layer.

Sprinkle the remaining 1/2 cup of shredded cheddar cheese (and the Monterey Jack, if using) evenly over the top of the casserole mixture.

Prepare the crunchy topping: In a small separate bowl, combine the crushed cornflakes or panko breadcrumbs with the 2 tablespoons of melted butter. Toss until the crumbs are evenly coated, then sprinkle the buttery crumbs over the cheese layer.

Place the casserole in the preheated oven and bake for 40 to 45 minutes, or until the sauce is bubbly around the edges, the hashbrowns are cooked through, and the topping is golden brown and crispy.

Remove the casserole from the oven. Let it rest for 5 to 10 minutes before slicing and serving. This allows the creamy base to set up properly, making serving much easier.