Preheat the oven to 350 degrees F. Lightly grease a 9x13-inch baking dish with butter or cooking spray and set aside.

Combine the shredded chicken, cream of chicken soup, and sour cream in a large mixing bowl. Stir until completely smooth and well-incorporated with no visible streaks of sour cream. The mixture should look thick and creamy throughout.

Spread the chicken mixture evenly into the prepared baking dish, smoothing the top flat with a spatula so it bakes evenly from edge to edge.

In a separate bowl, combine the crushed crackers with the melted butter. Stir until every crumb is coated and the mixture begins to clump together like coarse, buttery sand.

Sprinkle the cracker mixture evenly over the chicken layer, covering the entire surface. Do not press it down. Leaving it loose is what creates the crispiest topping. If you press it down, it will compact and steam instead of crisping.

Bake for 30 to 35 minutes until the casserole is bubbling around the edges and the cracker topping is deep golden brown. If the top starts to darken before the 30-minute mark, tent loosely with foil for the last 10 minutes. The internal temperature of the filling should reach 165 degrees F.

Remove from the oven and let the casserole rest for 5 minutes before serving so the filling sets and scoops cleanly on the plate.