Pour the buttermilk into one large bowl. In a second large bowl, mix together the flour, kosher salt, black pepper, garlic powder, onion powder, ground paprika, and cayenne pepper (optional). One piece at a time, dust the chicken thighs and drumsticks with the seasoned flour, then dip into the buttermilk. Dredge the chicken back in the flour mixture, making sure it’s thoroughly coated. Place the coated chicken pieces on a baking sheet, and cover them with wax paper, allowing them to rest until the flour coating is pasty, about 20-40 minutes. While the chicken is resting, preheat the cooking oil, in either a heavy pot on the stove or a deep fryer, to 375℉. Fry the chicken in batches until golden brown, about 7-8 minutes on each side. A thermometer inserted into the thickest part should read between 165-175℉. Remove and drain the chicken on paper towels before serving.