Preheat your oven to 375°F (190°C).
Cook the jumbo shells according to package instructions until al dente; drain and set aside.
In a large bowl, combine ricotta, mozzarella, Parmesan, egg, parsley, garlic powder, salt, and pepper.
Spread 1 cup of marinara sauce on the bottom of a baking dish.
Stuff each shell with the cheese mixture and place them in the dish.
Pour the remaining marinara sauce over the stuffed shells.
Sprinkle additional mozzarella cheese on top if desired.
Cover the dish with aluminum foil and bake for 20 minutes.
Remove the foil and bake for an additional 10 minutes until bubbly and golden.
Let cool for a few minutes before serving.