Butterfly the chicken breasts so you end up with 4 thinner pieces. In a bowl combine buttermilk, garlic powder, paprika, onion powder, chilli powder and all purpose seasoning. Add the chicken, mix well, cover and refrigerate for 2–3 hours or longer if possible.
Prepare the coleslaw by combining cabbage, red cabbage, broccoli stem, celery, carrots, red onion and herbs in a large bowl. Add Dijon mustard, mayonnaise, soured cream or yoghurt, apple cider vinegar, paprika, salt and pepper. Mix well and place in the fridge until ready to use.
In a shallow bowl combine cornstarch, plain flour, gochugaru, salt and pepper for the dry batter.
Remove the chicken from the marinade and allow excess marinade to drip off. Coat thoroughly in the dry batter, pressing the coating firmly onto the chicken to create texture. Shake off any excess.
Heat oil in a pot or deep pan over medium heat. Fry the chicken on medium–low heat for 7–8 minutes or until golden brown, crispy and fully cooked through. Remove and drain on paper towel.
In a bowl combine gochujang, light soy sauce, dark soy sauce, ketchup, honey and melted butter. Heat gently if needed and mix until smooth and glossy, then toss the hot fried chicken in the sauce until fully coated.
Lightly toast the brioche buns with a little butter until golden.
Assemble the burgers by adding a generous amount of coleslaw to the bottom bun, followed by the sauced crispy chicken and the top bun.
Serve immediately while hot and crispy.