Get the three coatings ready: Pour the flour and cornstarch onto a large plate and stir them together with a fork. Hold this aside. Combine the buttermilk and egg in a large, shallow bowl, whisking them well until the mixture is smooth. Hold it aside. Finally, pour the panko bread crumbs onto another large plate. Have these three dishes ready in a work area next to the seasoned chicken breasts and the prepared baking sheet.
First dredge a chicken cutlet with the flour-cornstarch mixture on both sides, tapping off the excess. Next, dip the chicken into the buttermilk-egg mixture to coat both sides. Finally, press the chicken into the panko bread crumbs until the whole surface is coated; press them firmly to make sure they stick. Place the breaded cutlet on the baking sheet. Repeat these steps with the rest of the cutlets.
Arrange the chicken on the baking sheet with space between each piece: this is important because crowding the chicken will create steam and prevent the breading from crisping up. You can always use two baking sheets if necessary.
