Make the buttermilk soak: in a large bowl or zipper bag, combine the buttermilk, cayenne pepper, paprika, mustard powder, garlic powder, salt, and pepper. Add the chicken breasts, cover with plastic wrap and refrigerate at least 2 hours or overnight.
Make the breading: in a shallow dish whisk together the flour, cornstarch, dried parsley, paprika, mustard powder, onion powder, salt, and pepper.
Prepare your breading station: take the chicken out of the fridge, have your breading ready and a separate plate to place the breaded chicken breasts. Remove the breasts one by one letting the excess marinade drip off, transfer to the flour mixture making sure to fully cover both sides, then transfer to a plate.
Once all the chicken pieces are breaded, let them rest for 10 mins and heat the oil in a large skillet over medium-high heat.
Test the oil by dropping a little flour into it. If it sizzles, the oil is ready. Fry the chicken breasts for 7-8 minutes on each side until golden brown and the internal temperature of the chicken is 170 F for breasts and 180 F for thighs.
Transfer the fried breast to a plate lined with a paper towel to drain the excess oil and serve while hot.