Preheat the oven to 375°F (190°C) and lightly spray a 9×13-inch baking dish with cooking spray; set aside.
Bring a large pot of salted water to a boil and cook the jumbo pasta shells according to package directions until al dente; drain and let cool enough to handle.
In a mixing bowl, combine the cooked chopped chicken, ricotta cheese, pesto, garlic powder, dried basil, and 1 cup of the shredded mozzarella until evenly mixed.
Spoon the chicken and ricotta mixture into each cooled shell and arrange the stuffed shells in the prepared baking dish in a single layer.
Pour the spaghetti sauce over the filled shells, then sprinkle the remaining 1 cup of shredded mozzarella evenly on top.
Bake uncovered for 25–30 minutes, until the sauce is bubbly and the cheese is melted and lightly golden; remove from oven and let sit a few minutes before serving.