For the honey mustard apple slaw, in a medium bowl, whisk together the buttermilk, yogurt, honey, mustard, salt, and pepper. Stir in the coleslaw mix and apple. Cover and refrigerate until ready to serve. The mild acidity of the yogurt and buttermilk will cause the cabbage (the coleslaw mix) to soften and wilt, therefore releasing a good amount of its moisture. Toss the slaw thoroughly before serving, and feel free to drain any thin liquid that has pooled in the bowl before serving if you make it ahead of time!
In a large resealable plastic bag, combine the chicken breasts and thighs with the buttermilk, tossing to coat all of the chicken. Seal the bag and refrigerate, turning the bag once or twice if you can, for at least 1 hour and up to 24 hours.
Preheat the oven to 375 degrees F. Set a wire cooling rack on top of a large rimmed baking sheet and coat it with cooking spray.
In a large shallow dish, stir to combine the corn flakes, Old Bay, garlic powder, onion powder, salt, and pepper. Remove the chicken from the buttermilk, shaking off the excess. Discard the buttermilk. Dredge each piece of chicken in the cereal mixture, turning to coat both sides. Place the chicken breast on the prepared wire rack. Repeat the coating process with the remaining chicken pieces. Spray the chicken well with oil. Transfer the pan to the oven and bake until crispy, golden brown, and cooked through, 35 to 40 minutes.
Transfer the chicken to a serving platter and serve 1 piece of chicken (1 breast or 1 thigh) with about ½ cup slaw.