Boil a large pot of salted water. Cook the manicotti shells for about 6-8 minutes, or until al dente. Drain and set aside to cool slightly.
In a medium bowl, combine ricotta cheese, shredded mozzarella, grated Parmesan, egg, basil, garlic powder, salt, black pepper, and Italian seasoning. Mix until well combined.
Carefully stuff each manicotti shell with the cheese mixture using a spoon or a piping bag.
Spread 1 cup of marinara sauce on the bottom of a 9×13 inch baking dish.
Place the filled manicotti shells in the baking dish over the sauce.
Pour the remaining marinara sauce over the shells, ensuring they are well-covered.
Sprinkle extra mozzarella cheese on top, if using.
Cover the baking dish with aluminum foil to prevent over-browning.
Bake in the preheated oven for 30 minutes. Remove the foil and bake for an additional 15 minutes, until the cheese is bubbly and golden.
Allow the manicotti to cool for about 15 minutes before serving.
Notes
You can add cooked ground meat or sautéed spinach to the cheese filling for added protein and flavor.