In a medium bowl, whisk together the gochujang, soy sauce, honey, sesame oil, minced garlic, grated ginger, rice vinegar, and black pepper until well-blended.

Coat the chicken thighs with the marinade and refrigerate for at least 1 hour, preferably overnight.

In a large bowl, combine the green cabbage and carrots. In a separate bowl, whisk together the mayonnaise, lime juice, and optional sriracha until creamy. Pour over the cabbage and carrots and toss.

Preheat a grill pan or skillet over medium-high heat and add oil. Grill chicken thighs for 6–7 minutes on each side until internal temperature reaches 165°F (75°C).

Let the chicken rest for 5 minutes, then slice into strips.

Toast the brioche buns for 2–3 minutes until golden brown. Assemble the sandwich by layering sliced chicken and crispy slaw, garnishing with cilantro and sesame seeds.