Preheat oven to 375°F (190°C). Lightly grease a baking dish and set aside
Bring a large pot of salted water to a boil. Cook manicotti shells according to package directions until al dente. Drain and let cool slightly.
In a large bowl, combine the cooked chicken, ricotta cheese, ½ cup of Parmesan, ½ cup of mozzarella, garlic, Italian seasoning, salt, pepper, and fresh basil. Mix until well combined. Mix until well combined.
In a separate bowl, stir together the marinara sauce and heavy cream to create a creamy tomato sauce.
Spoon a thin layer of sauce into the bottom of the prepared baking dish.
Carefully fill each manicotti shell with the chicken and ricotta mixture and place seam-side down in the dish
Pour the remaining sauce evenly over the filled manicotti.
Sprinkle the remaining mozzarella and Parmesan cheese over the top.
Cover loosely with foil and bake for 20 minutes. Remove foil and bake an additional 10 minutes, until bubbly and lightly golden.
Remove from oven and let rest for 5 minutes before serving.