In a medium bowl, whisk together soy sauce, brown sugar, honey, gochujang, rice vinegar, sesame oil, garlic, ginger, sesame seeds, and green onions.
Add the chicken and toss until fully coated in the marinade.
Marinate for 20 minutes at room temperature or refrigerate for up to 2 hours for deeper flavor.
In a large bowl, combine the green cabbage, red cabbage, and carrot.
In a small bowl, whisk together mayonnaise, rice vinegar, sugar, salt, and pepper.
Pour the dressing over the cabbage mixture and toss well to combine.
Refrigerate the slaw until ready to serve.
Heat a cast-iron skillet or grill pan over medium-high heat. Lightly oil if needed.
Cook the chicken for 6–7 minutes per side until caramelized and cooked through, reaching an internal temperature of 165°F (74°C).
Let the chicken rest for 5 minutes before slicing.
Split the buns and lightly butter each side.
Toast the buns in a skillet or under a broiler until golden brown.
Assemble the Korean BBQ Chicken Sandwich by placing the chicken on the bottom bun.
Spoon over extra sauce if desired.
Top with a generous portion of cabbage slaw.
Finish with the top bun and serve immediately.