Preheat the oven to 400°F (200°C).
Place each chicken breast between two sheets of plastic wrap and pound to an even thickness of about 1/2 inch using a meat mallet or rolling pin.
In a shallow bowl, combine the whole wheat breadcrumbs, grated Parmesan cheese, dried Italian seasoning, garlic powder, salt, and black pepper.
In a separate shallow bowl, whisk together the egg and water.
Dip each chicken breast first into the egg mixture, allowing excess to drip off, then coat evenly with the breadcrumb mixture. Press the breadcrumbs onto the chicken to adhere well.
Heat the olive oil in a large oven-safe skillet over medium heat. Add the breaded chicken breasts and cook for 3-4 minutes on each side until golden brown.
Remove the skillet from heat. Spoon 1/4 cup marinara sauce over each chicken breast, then evenly sprinkle 1 ounce of shredded mozzarella cheese on top of each.
Transfer the skillet to the preheated oven and bake for 10-12 minutes, or until the chicken is cooked through and the cheese is melted and bubbly.
Remove from the oven and let rest for 2 minutes. Garnish with fresh basil leaves before serving.