If using 2 large chicken breasts, using a sharp knife, slice each chicken breast in half lengthwise to create two thin fillets with each. Once you have four filets, place each on a cutting board, cover with plastic wrap, and use a meat tenderizer to pound until each filet is about ½" thick.
Preheat oven to 400 degrees and set a baking rack over a foil or parchment paper covered baking sheet. Lightly brush rack with cooking oil.
Melt butter in a medium skillet over medium heat. Add panko and stirring frequently, cook until golden brown, about 5 minutes.
Transfer panko to a shallow bowl and mix with garlic powder, paprika, dried parsley, ½ tsp salt, and ½ tsp pepper.
In a small bowl, mix together mayonnaise and dijon mustard.
Sprinkle remaining ½ tsp salt and ½ tsp pepper over both sides of chicken breasts and brush both sides with mayo mixture. Dredge each chicken breast in panko mixture, pressing down to adhere to chicken and being sure to coat evenly. Place chicken breasts on prepared wire rack.
Bake for 15-22 minutes, until internal temperature reaches 165 degrees.
Serve panko crusted chicken with parsley and lemon, if desired.