Preheat the oven to 350°F.
Lightly grease a 9x13-inch baking dish with nonstick cooking spray or butter.
In a large mixing bowl, combine the cream of chicken soup, sour cream, poppy seeds, freshly grated Parmesan cheese, garlic powder, and black pepper.
Stir until the mixture is smooth and evenly combined.
Add the cooked chicken to the creamy mixture.
Stir until the chicken is fully coated.
Transfer the chicken mixture to the prepared baking dish.
Spread it into an even layer.
In a medium bowl, combine the crushed butter crackers and melted butter.
Stir until the cracker crumbs are evenly coated.
Sprinkle the buttered cracker crumbs evenly over the chicken mixture.
Bake uncovered for 30 minutes, or until the casserole is hot, bubbly, and the cracker topping is golden brown.
Remove the casserole from the oven.
Allow it to rest for several minutes before serving.
Serve warm over cooked white rice.
If desired, cooked rice may also be spread in the bottom of the baking dish before adding the chicken mixture.