In a large bowl, combine soy sauce, brown sugar, honey, gochujang, rice vinegar, sesame oil, minced garlic, grated ginger, and toasted sesame seeds, whisking until smooth. Add the chicken thighs and marinate for at least 20 minutes, or up to 2 hours.

In a separate bowl, mix together the green cabbage, red cabbage, and julienned carrot. Whisk mayonnaise, rice vinegar, sugar, salt, and pepper in another bowl, then pour over the cabbage mixture and toss well before refrigerating.

Heat a cast-iron skillet over medium-high heat. Cook the marinated chicken thighs for 6-7 minutes on each side until cooked through, then remove from heat and let rest for 5 minutes.

Split the buns, spread butter on the cut sides, and toast them in the same skillet or under the broiler until golden brown, about 2-3 minutes.

Layer the grilled chicken on the bottom bun, drizzle with BBQ sauce if desired, then add the cabbage slaw and top with the other half of the bun before serving.