Place chicken breast on a cutting board. With your palm on top, cut horizontally through the thickest part, leaving ½ inch uncut. Open like a book. Cover with cling wrap and gently pound with a meat mallet or rolling pin until evenly flat. Repeat with remaining pieces.
Marinate
Combine marinade ingredients in a bowl.
Add chicken, turning to coat evenly. Cover and refrigerate at least 2 hours, preferably overnight.
Preheat oven
Remove chicken from fridge while oven preheats to 420 F (215 C) for conventional oven and 390 F (200 C) for convection oven. Line one baking sheet with foil. Place a wire rack on top if available (highly recommend). Line another baking sheet with foil to toast the bread crumb
Toast the panko
Toss panko with oil and spread evenly on a baking sheet. Bake for about 5 minutes, until lightly golden. Set aside to let it cool down
Prepare the batter:
Whisk egg, mayonnaise, cornstarch, and salt until smooth.
Batter and coat the chicken:
Dip chicken in batter, then press firmly into toasted panko on both sides.
Bake
Spray one side of chicken with oil, place oil-side down on rack, then spray the top. Repeat with the rest of the chicken pieces.
Bake for 15-20 minutes (depending on the size of the chicken), or until internal temperature reaches 165 F (74 C).
Finish
Combine the finishing spice mix. Sprinkle it over the chicken pieces. Let rest 10 minutes on the rack before slicing into cutlets and serve immediately