Preheat your oven to 375°F (190°C).
Cook manicotti shells according to package instructions until al dente. Drain and set aside to cool.
In a mixing bowl, combine ricotta cheese, mozzarella cheese, Parmesan cheese, egg, parsley, oregano, garlic powder, salt, and pepper. Mix until well combined.
Using a spoon or piping bag, carefully fill each manicotti shell with the cheese mixture.
Spread a thin layer of marinara sauce on the bottom of a baking dish. Place the filled manicotti shells in the dish.
Pour the remaining marinara sauce over the top of the manicotti.
Sprinkle with additional Parmesan cheese for garnish.
Cover the baking dish with foil and bake for 25-30 minutes, or until the sauce is bubbly and the cheese is melted.
Remove foil and bake for an additional 5-10 minutes until the cheese is golden brown.
Serve hot and enjoy!