Preheat oven to bake 375 degrees F. Tip: Place cookie sheet in oven when you preheat. The casserole will cook faster.
Mix together the cracker crumbs, melted butter and poppy seed in a bowl. Layer a 6 x 8 baking dish with 2/3 of the cracker mix. Thoroughly combine the soup, sour cream and chunky chicken together and spoon the mixture over the crumb crust. Sprinkle the remaining 1/3 of the cracker mix over the top of the chicken mixture.
Bake in the preheated oven until the casserole is hot and bubbling and the topping is crispy brown, about 1 hour.