In a bowl, combine the buttermilk and chicken breasts. Let them marinate for about 15 minutes.
Mix the flour, panko breadcrumbs, salt, pepper, paprika, garlic powder, and onion powder in a shallow dish.
Heat vegetable oil in a deep pan to 350°F (175°C).
Dredge each marinated chicken breast in the flour mixture, ensuring an even coating.
Fry the chicken breasts for 5-6 minutes on each side until golden brown and cooked through. Remove and place on a paper towel-lined plate.
Whisk together the Korean-style BBQ sauce, sesame oil, mayonnaise, rice vinegar, and honey in a small bowl.
Toss the coleslaw mix in the sauce mixture until well coated.
Toast the hamburger buns.
To assemble, spread the BBQ sauce on the bottom bun, place the fried chicken on top, then add the coleslaw mixture. Sprinkle with sesame seeds.
Cover with the top bun and secure with a toothpick if needed.
Serve the Korean Fried Chicken Sandwich immediately.