Cut each chicken breast in half width-wise so you have 4 chicken cutlets. Gently pound chicken cutlets flat to about 1/2-inch thickness between two sheets of wax paper using a meat mallet or a rolling pin.
In a shallow bowl, whisk egg and milk together until smooth. Place flour in a separate shallow bowl and season with salt and pepper. Place panko, Italian seasoning, garlic powder, and onion powder in third shallow bowl and season with salt and pepper.
Dredge chicken cutlets in flour, shaking off excess. Dip chicken in egg, then roll in panko until fully coated. Place cutlets on a plate.
Heat a large skillet over medium-high heat. Place 2 tablespoons butter and 1 tablespoon olive oil in skillet once hot and swirl to evenly coat pan. Add chicken in a single layer to pan (work in batches if needed, using additional butter and olive oil) and sear 3-4 minutes on first side. Flip chicken, add remaining 1 tablespoon butter, and cook an additional 2-3 minutes until chicken registers 165F. Transfer chicken to a clean plate.
Remove pan from heat and allow to cool slightly. Nestle chicken cutlets back into pan in a single layer. Spoon about 1/4 cup of warmed marinara sauce over chicken and top with fresh mozzarella and Parmesan.
Broil chicken Parmesan 2-3 minutes on high several inches from heat until cheese is melted and lightly browned. Serve chicken over hot spaghetti with additional marinara and fresh basil on top. Enjoy!