Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the cooked chicken, broccoli, cream of mushroom soup, milk, garlic powder, pepper, and half of the shredded cheddar cheese.
Transfer the mixture to a greased 9x13 inch baking dish and spread it out evenly.
Sprinkle the remaining cheddar cheese over the top of the casserole.
In a small bowl, combine breadcrumbs and melted butter, mixing until evenly coated.
Sprinkle the buttery breadcrumbs over the cheese layer.
Cover the casserole with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 15 minutes until the top is golden and bubbly.
Let it cool for a few minutes before serving.