Place all chicken pieces in a large bowl or zip-top bag and pour buttermilk over them until fully submerged. Marinate for at least 15 minutes, or up to overnight in the refrigerator for best tenderness.

In a shallow bowl or pie dish, combine flour, cornstarch, paprika, garlic powder, onion powder, cayenne pepper, black pepper, and salt. Whisk together thoroughly to distribute spices evenly.

Fill a large, heavy-bottomed pot or Dutch oven with oil about 3 inches deep. Heat to 350 degrees Fahrenheit, using a thermometer to ensure accuracy.

Remove each chicken piece from buttermilk, letting excess drip off. Dredge thoroughly in the flour and cornstarch mixture, coating all sides evenly and pressing the coating to help it adhere. Place coated pieces on a wire rack set over a baking sheet and let rest for 5 minutes.

Carefully lower chicken pieces into hot oil, working in batches to avoid overcrowding. Fry drumsticks for 12 to 15 minutes and thighs for 14 to 17 minutes until coating is deep golden brown and internal temperature reaches 165 degrees Fahrenheit. Transfer to paper towel-lined plate to drain.