Place the chicken thighs and drumsticks in a large shallow dish. Mix together the buttermilk, garlic, lemon juice and black pepper and pour over the chicken. Turn the chicken portions to coat in the buttermilk and cover and marinate in the fridge for at least 1 hour.
Preheat the oven to 200°C/gas mark 6. Mix the crushed cornflakes with the oregano and the cayenne pepper and spread out on a large flat plate.
Lift each chicken portion from the marinade, shaking off any excess. Roll in the cornflake mixture, pressing down well to completely coat. Place on a large baking tray lined with greaseproof paper.
Bake in the oven for 35-40 minutes until the chicken is cooked through (the juices should run clear when the flesh is pierced with the tip of a knife) and the cornflake crust is crisp and golden. Cover loosely with foil if the crust starts to over-brown before the chicken is cooked.
Mix the mayonnaise with the chives in a small bowl. Serve the chicken with the chive mayonnaise.