Preheat the oven to 375°F (190°C).
In a large mixing bowl, combine the shredded chicken and barbecue sauce.
Add the diced onion, garlic powder, salt, and pepper to the chicken mixture; stir until well combined.
In another bowl, combine the cornbread mix and milk until just mixed.
Layer the chicken mixture in a greased baking dish, then pour the cornbread batter over the top evenly.
Sprinkle the shredded cheddar cheese on top of the cornbread batter.
Bake for 25-30 minutes or until the cornbread is golden and cooked through.
Remove from oven and let cool for a few minutes; serve warm.