Slice each chicken breast in half horizontally to create two thin cutlets. Place the cutlets between two slices of plastic wrap and pound to a ¼-inch thickness. Season lightly with salt and pepper.
Create a breading station with three medium bowls. Place the flour in one bowl, the beaten eggs in a second bowl, and then combine the panko, ¼ cup Parmesan, and garlic powder in a third bowl.
Working with one cutlet at a time, use a fork to dip it in the flour, and then into the egg, letting any excess drip off into the bowl. Finally, dip the chicken into the panko mixture, turning to coat it completely. Place it on a baking sheet and repeat with the remaining cutlets.
Heat one side of the Blackstone (or other outdoor griddle) to medium-high. Pour the oil in an even layer, then add the chicken cutlets and cook until the bottom is golden brown, 4 to 5 minutes.
Flip the cutlets over and top each one with 2 tablespoons marinara sauce. Sprinkle the mozzarella and remaining ¼ cup Parmesan cheese evenly over the top of the chicken.
Cover the griddle and cook until the cheese is melted and the chicken has reached an internal temperature of 165°F (74°C), about 8 minutes.
Remove from the griddle and serve hot.
Notes
Get ahead--You can bread the chicken up to 30 minutes before cooking. Keep it in the fridge on a wire rack set over a baking sheet. This will help to prevent the coating from becoming soggy.
Warm your sauce--Make sure that the marinara sauce is warm before spooning it on the chicken. You can heat it in a small pot on the other side of the griddle while the chicken is cooking or on a stovetop or camp stove.