Preheat the oven to 350°F (175°C).
Cook the pasta shells according to package instructions until al dente. Drain and set aside.
In a large bowl, combine the ricotta cheese, mozzarella cheese, Parmesan cheese, egg, parsley, garlic powder, salt, and pepper. Mix until well combined.
Stuff each cooked shell with the cheese mixture.
Spread a thin layer of marinara sauce on the bottom of a baking dish.
Arrange the stuffed shells in the baking dish and cover with the remaining marinara sauce.
Sprinkle extra mozzarella and Parmesan cheese on top if desired.
Cover the baking dish with aluminum foil and bake for 25-30 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let it cool for a few minutes before serving.