Preheat the oven to 375°F (190°C) and lightly spray a 9×13-inch baking dish with cooking spray.
Cook the jumbo pasta shells in a large pot of boiling salted water according to package directions until al dente, then drain and let cool slightly so they can be handled.
In a mixing bowl, combine the cooked chopped chicken, ricotta cheese, pesto, 1 cup of the shredded mozzarella, garlic powder, and dried basil until evenly mixed.
Spoon or pipe the chicken-ricotta mixture into each cooled shell and arrange the stuffed shells in the prepared baking dish in a single layer.
Pour the spaghetti sauce evenly over the stuffed shells, then sprinkle the remaining 1 cup of shredded mozzarella on top.
Bake uncovered for 25–30 minutes, until the sauce is bubbly and the cheese is melted and lightly golden.