Preheat oven to 350°F and lightly grease a 9x13-inch baking dish.
Mix cream of chicken soup, sour cream, milk, salsa, garlic powder, salt, and pepper until smooth.
Fold in cooked chicken and half of the cheddar cheese.
Lightly crush some Doritos into bite-size pieces and spread them across the bottom of the dish.
Spread the creamy chicken mixture evenly over the Doritos.
Add another layer of lightly crushed Doritos and sprinkle the remaining cheddar over the top.
Bake uncovered for 25–30 minutes, until hot, bubbling, and the cheese is melted.
For extra crunch, add reserved Doritos during the final 5 minutes of baking.
Let the casserole rest for 5 minutes.
Serve warm with optional fresh toppings such as green onions, tomatoes, or salsa.