Place buttermilk in a 2-quart baking dish; place fillets in buttermilk and allow to marinate for 15 minutes.
Place oil in a 12 incg cast iron skillet to a depth of about 2 inches. Heat until oil registers 375 degrees F. on an instant-read thermometer.
In a large, zipper-lock food storage bag, mix remaining ingredients by zipping them inside bag and shaking until well blended. Remove fillets from buttermilk; allow excess liquid to drain.PPlace fillets in bag and shake gently until completely coated.
Gently shake off excess coating mixture and carefully place into hot oil. Fry until golden brown, about 5 minutes on each side. Drain on paper towels and serve.