Pat the chicken pieces dry with kitchen paper, then place them in a large bowl or resealable bag. Pour over the buttermilk, cover and chill in the fridge for at least 2 hours, or preferably overnight.
In a large, shallow dish or paper bag, whisk together the plain flour, salt, black pepper, paprika, garlic powder, onion powder and cayenne pepper.
Remove the chicken from the buttermilk, letting any excess drip off. Dredge each piece in the seasoned flour, pressing firmly to coat. Arrange the coated pieces on a wire rack set over a baking tray and let them rest for 15–20 minutes.
Pour enough sunflower oil or lard into a heavy-based frying pan to a depth of about 2.5–3cm. Heat over a medium–high heat until it reaches 175°C, or until a pinch of flour sizzles when dropped in.
Working in batches, fry the chicken skin-side down for 6–8 minutes per side, until deeply golden. Reduce the heat to medium–low, cover the pan and cook for another 10–15 minutes, or until the chicken is cooked through and a thermometer reads 74°C.
Transfer the chicken to a wire rack set over kitchen paper to drain. Leave to rest for a few minutes before serving.