Combine the chicken, cheese, sour cream, milk, green onions, poultry seasoning, garlic powder, salt, and pepper in a large bowl. Stir until completely combined.
Transfer the mixture to the prepared baking dish, spreading it into an even layer.
Whisk the cornbread mix, corn, eggs, and butter in another mixing bowl. Pour evenly over the chicken mixture.
Sprinkle additional cheese on top, if desired.
Bake for 35-45 minutes or until a toothpick test returns clean and the cornbread topping is golden brown.
Let the casserole cool at room temperature for a few minutes to set before slicing and serving.
Notes
Optional Additions and Variations:Add chopped green chilies or jalapeños to the filling. Mix in crumbled bacon. Top with a dollop of sour cream or a drizzle of hot sauce. Use a combination of different cheeses.