Preheat your oven to 375°F (190°C).
Cook the jumbo pasta shells according to package instructions until al dente. Drain and set aside to cool slightly.
In a large skillet over medium heat, brown the ground beef until fully cooked. Drain excess fat.
In a mixing bowl, combine the cooked beef, ricotta cheese, 1/2 cup of mozzarella cheese, Parmesan cheese, egg, Italian seasoning, garlic powder, salt, and pepper. Mix until well combined.
Spread 1 cup of marinara sauce evenly on the bottom of a 9x13 inch baking dish.
Stuff each pasta shell with the beef mixture and place them in the baking dish, open side up.
Pour the remaining marinara sauce over the stuffed shells, then sprinkle the remaining mozzarella cheese on top.
Cover the baking dish with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Let cool for a few minutes before serving. Garnish with fresh basil if desired.