In a large mixing bowl or container, blend together the buttermilk, soy sauce, salt, pepper, onion granules and garlic powder for the marinade.
Add the chicken pieces to the marinade and mix thoroughly. Put in the fridge to tenderise for minimum one hour.
Mix together the Korean Gochujang sauce ingredients. Set aside.
When the chicken is ready, add vegetable oil to a large frying pan on high heat. You want about an inch of oil, enough to predominantly cover the chicken.
Mix together the flour, 1 1/2 tsp of salt and 1 tsp of pepper. Remove the chicken from the buttermilk one by one, allowing the excess to drip off, and dredge through the flour. You want a nice even coating, and to shake off any excess.
When the oil is very hot, add the chicken to the pan and cook for 7-10 minutes until golden brown and cooked through. Turn chicken halfway through frying. I'd recommend cooking in batches so as not to crowd the pan, and to change the oil halfway if it becomes discoloured.
While the chicken is frying, heat the sauce ingredients in a small saucepan until blended. Add the sesame seeds to another saucepan on medium heat for a minute or two to toast, take off the heat when they start to brown.
As pieces of chicken are cooked, remove from the pan and set aside on a kitchen town lined plate or bowl to soak up the excess oil. Once all the chicken is cooked, move it to a large bowl and cover in the Gochujang sauce. Mix until evenly coated.
Served topped with sesame seeds, sliced green onion and red chillis.