In a large bowl, whisk together soy sauce, brown sugar, honey, gochujang, rice vinegar, sesame oil, minced garlic, and grated ginger until well combined. Add the boneless skinless chicken thighs, ensuring they are fully coated in the marinade. Cover the bowl and let the chicken marinate for at least 20 minutes at room temperature, or refrigerate for up to 2 hours for deeper flavors.

While the chicken is marinating, prepare the cabbage slaw by shredding green and red cabbage and mixing them in a bowl. In a separate small bowl, whisk together mayonnaise, rice vinegar, and sesame oil until smooth. Combine the dressing with the cabbage mixture, tossing thoroughly to coat. Place the slaw in the fridge to chill and develop its flavors.

Preheat a skillet or grill pan over medium-high heat. Remove the marinated chicken thighs from the marinade and place them in the preheated pan. Cook for about 6-7 minutes on each side until nicely charred and cooked through.

In the meantime, butter the cut sides of the buns. In the same skillet, place the buns cut side down and toast them until golden brown and crispy, about 2-3 minutes.

To assemble the sandwich, layer the sliced chicken on the bottom bun, drizzle with BBQ sauce if desired, and top with the chilled cabbage slaw. Finally, place the top bun over the slaw.