Set oven to 350 degrees. Cook pasta shells according to package directions. Drain and rinse with cold water.
To a large bowl add ricotta cheese, 1 1/2 cup of mozzarella, and 1/2 cup of Parmesan, egg, parsley, salt, and garlic powder. Mix together.
Spoon 3/4 of marinara sauce on the bottom of a 9x13 inch baking dish. Stuff each pasta shell with cheese filling and place on top of sauce. Drizzle remaining sauce over the stuffed shells and top with the remaining mozzarella cheese. Cover with foil and bake for 20 minutes then uncover and bake for an additional 10 minutes until cheese is golden brown.
Remove from oven and sprinkle with remaining Parmesan cheese. Serve immediately.