Preheat the oven to 400 degrees F. Set a wire rack over a baking sheet and spray with non-stick spray.
Sprinkle the chicken breasts with salt and pepper. Set up a breading station with three baking dishes or pie plates. In the first, mix the flour with a generous amount of salt and pepper. In the second, whisk together the egg whites and milk. In the third, mix the breadcrumbs with the Parmesan.
Bread each chicken breast by first coating in the flour, shaking off the excess; then dunking in the egg whites; and, finally, dredging in the breadcrumb mixture, again shaking off the excess. Place the breaded chicken on the prepared wire rack.
Bake until the chicken is cooked through and the coating is browned, 15 to 20 minutes. Top each breast with 1/4 cup of the marinara sauce and 2 slices of mozzarella cheese.
Return to oven and bake until the cheese is bubbling, another 5 to 10 minutes. Garnish with basil leaves, and serve.