Marinate Chicken: Place mixed chicken pieces in a bowl. Add buttermilk, hot sauce, and chilli oil. Massage with hands to coat each piece thoroughly. Cover and refrigerate for 10 to 24 hours.
Prepare Coating Mix: In a separate bowl, combine all-purpose flour, cornstarch, celery seed, paprika, chilli powder, black pepper, onion powder, garlic powder, and salt. Mix well.
Coat Chicken: Remove chicken from marinade and coat each piece evenly with the flour mixture. Place coated chicken on a rack and let rest to absorb the coating.
First Fry: Heat vegetable oil in a deep fryer or pot to 160°C. Fry chicken pieces without overcrowding for 8 minutes.
Rest and Increase Heat: Remove chicken and let rest for 7 minutes. Increase oil temperature to 190°C.
Second Fry: Return chicken to hot oil and fry for an additional 3 minutes to achieve extra crispiness.
Season and Serve: While hot, season chicken with homemade chicken salt or preferred seasoning. Serve immediately.