In a large mixing bowl, whisk together soy sauce, brown sugar, honey, gochujang, rice vinegar, sesame oil, minced garlic, grated ginger, and toasted sesame seeds. Add the chicken thighs, ensuring they are coated in the marinade. Cover and let marinate for at least 20 minutes or refrigerate for up to 2 hours.

Combine the shredded green and red cabbage along with the julienned carrot in a large bowl. In a separate bowl, whisk together mayonnaise, rice vinegar, sugar, salt, and pepper. Pour the dressing over the cabbage mixture and toss to coat. Chill until ready to assemble.

Preheat a skillet or grill pan over medium-high heat. Remove the chicken from the marinade and cook for 6-7 minutes on each side, until charred and cooked through. Transfer to a plate and let it rest for 5 minutes before slicing.

Slice the buns in half and spread butter on each half. Toast them in the skillet until golden brown and crispy.

Layer sliced grilled chicken on the bottom half of each toasted bun. Drizzle with extra BBQ sauce if desired. Top with cabbage slaw and the top bun.