In a medium bowl, combine the gochujang, soy sauce, honey, sesame oil, garlic, ginger, rice vinegar, and black pepper. Whisk until well blended and set aside.

Place the chicken thighs in a resealable bag or bowl, coating them evenly with the marinade. Cover and refrigerate for at least 1 hour, ideally overnight.

In a large mixing bowl, combine the shredded cabbage and carrots, tossing to mix evenly.

In a small bowl, whisk together the mayonnaise, lime juice, and optional sriracha. Pour over the cabbage and carrots, tossing well to coat. Chill in the refrigerator.

Preheat a grill pan or skillet over medium-high heat with a drizzle of oil. Grill the marinated chicken thighs for 6–7 minutes per side until cooked through.

Remove the chicken from the heat and let it rest for about 5 minutes before slicing.

Toast the brioche buns until golden brown.

Assemble the sandwiches by layering grilled chicken and slaw on the bottom half of the buns. Top with cilantro and sesame seeds, then cap with the top half of the bun.