Preheat the oven to 400°F (200°C). Lightly grease a baking dish with olive oil cooking spray.
Place the chicken breasts between two sheets of plastic wrap and gently pound them to an even thickness, about 1/2 inch thick.
In a shallow bowl, combine the breadcrumbs, grated Parmesan cheese, Italian seasoning, garlic powder, salt, and pepper.
In another shallow bowl, place the beaten egg.
Dip each chicken breast first in the egg, allowing any excess to drip off, then dredge in the breadcrumb mixture, pressing to adhere. Place the coated chicken breasts in the prepared baking dish.
Spray the tops of the chicken breasts lightly with olive oil cooking spray.
Bake in the preheated oven for about 20 minutes.
While the chicken is baking, warm the marinara sauce in a small saucepan over low heat.
After 20 minutes, remove the chicken from the oven. Spoon the marinara sauce over the chicken breasts, then top each with shredded mozzarella cheese.
Return to the oven and bake for an additional 5-10 minutes, or until the cheese is melted and bubbly, and the chicken reaches an internal temperature of 165°F (75°C).
Remove from the oven and let it rest for a few minutes before serving.