Preheat oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish.
In a large bowl, combine cooked chicken, cream of chicken soup, sour cream, milk, melted butter, garlic powder, salt, and pepper. Mix well.
Fold in the frozen vegetables, if using, then spread the mixture evenly into the prepared baking dish.
In another bowl, combine stuffing mix with chicken broth. Let sit for 5 minutes to absorb the liquid.
Fluff the stuffing with a fork and spread it evenly over the chicken mixture.
Cover with foil and bake for 25 minutes.
Remove foil and bake an additional 10-15 minutes until the top is golden and the casserole is bubbling.
Let sit for 5 minutes before serving.