Bring a large pot of salted water to a boil. Cook the pasta shells according to package directions until al dente. Drain and rinse under cold water to stop cooking. Set aside.
In a large bowl, combine shredded chicken, ricotta, mozzarella, Parmesan, pesto, egg, garlic powder, salt, and pepper. Mix until fully incorporated.
Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish. Spread 1 cup of sauce on the bottom of the dish.
Carefully fill each shell with 1–2 tablespoons of the pesto chicken mixture and arrange them in the dish.
Spoon the remaining sauce over the stuffed shells and sprinkle with extra mozzarella on top.
Cover the baking dish with foil and bake for 20 minutes. Uncover and bake for an additional 10 minutes, or until the cheese is bubbly and slightly golden.
Let cool for 5 minutes before serving. Garnish with fresh basil or parsley if desired.