Combine the buttermilk, egg, salt, pepper, garlic powder and cayenne pepper in a measuring jug or bowl.
Place the chicken pieces in a ziploc bag and pour over the brine. Seal the bag then place in the fridge for at least 4 hours but ideally up to 24 hours for the juiciest end result.
Add flour, cornstarch, baking powder, a few teaspoons of salt, pepper, garlic powder, onion powder, cayenne pepper, smoked paprika, dried oregano and celery salt in a large bowl.
Add enough oil to deep fry the chicken (approximately 8 cups) to a pot set over medium-high heat. The oil is ready when a wooden spoon is pressed to the bottom of the pot and bubbles appear around the edges of the spoon.
Add 4 tablespoons of brine to the flour mixture, mixing lightly to create a few clumps in the flour.
Wipe the excess brine from the chicken and coat in the flour, making sure the chicken is thoroughly coated.
Shake off excess flour then carefully lower into the hot oil.
Allow the chicken to fry for 4-5 minutes per side until the chicken is golden brown and crisp.
Remove the chicken from the oil then place on a wire rack set over a sheet pan.
Place the chicken in a hot oven (200ºC/390ºF) for 7-10 minutes until the chicken is cooked through (the flesh will be opaque white to the bone or read 165ºF/73ºC on a meat thermometer).
Serve the chicken with side dishes of your choice.