Preheat the Oven:
Preheat your oven to 350°F (175°C).
Prepare the Filling:
In a large mixing bowl, combine the cooked chicken, cream of chicken soup, cream of celery soup, chicken broth, sour cream, poultry seasoning, garlic powder, onion powder, and black pepper. Mix well until everything is thoroughly combined.
Assemble the Casserole:
Spread the chicken mixture evenly in the bottom of the 9x13 inch baking dish.
Prepare the Dressing:
In a medium bowl, mix the cornbread stuffing mix with the melted butter. Stir until the stuffing is evenly coated with butter.
Add the Dressing:
Sprinkle the stuffing mixture evenly over the chicken mixture in the baking dish.
Bake:
Cover the baking dish with aluminum foil and bake in the preheated oven for 30 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the top is golden brown and the casserole is bubbly.