1. Make the Buttermilk Soak: In a large bowl, combine the buttermilk with the salt, black pepper and cayenne. Add the chicken and stir to coat. Let stand at room temperature for 2 hours or refrigerate for 4 hours.
2. Make the Flour Dredge: In a large, re-sealable plastic bag, mix the flour with the onion powder, garlic powder, salt, black pepper and cayenne and shake to combine.
3. Set a rack on a baking sheet. Working with one piece at a time, remove the chicken from the buttermilk soak, letting the excess drip back into the bowl. Dredge the chicken in the flour mixture, pressing so it adheres all over. Transfer the coated chicken to the rack and let stand for 30 minutes.
4. In a deep skillet, heat 1 inch of oil to 350°. Set another rack over a baking sheet; line with paper towels. Fry the chicken in batches at 315°, turning once, until golden and an instant-read thermometer inserted in the thickest part of each piece registers 160°, 15 to 18 minutes. Transfer the chicken to the paper-lined rack and let stand for 5 minutes before serving.