Make a large incision in the thicker end of each chicken breast. Insert a piece of sliced cheddar. Season the buttermilk with 1 tbsp salt and dunk the stuffed chicken breasts in there. Marinade for 1+ hours.
Mix together your spice mix. Add all but 2 tbsp into the flour. Drain each chicken breast of a little buttermilk and dunk into the seasoned flour. Cover both sides of the chicken. Fry in hot oil until golden and crisp and the cheese is started to melt out of the coating. If you need to continue cooking the chicken in order to reach 70°C, transfer to a very hot oven for 5-10 minutes.
Make your sauce. Melt the butter in a small pan over low-medium heat. Add spices and tomato puree. Cook for 30 seconds or so, then chuck in the tomato ketchup, vinegar, worcestershire sauce and brown sugar. Heat until bubbling. Remove from the heat.
Slap your BBQ sauce all over your chicken pieces, then load into grilled brioche buns with lots of creamy slaw.