In a mixing bowl, combine gochujang, soy sauce, honey, sesame oil, minced garlic, grated ginger, rice vinegar, and a sprinkle of black pepper. Stir until smoothly blended.

Coat the chicken thighs in the marinade, ensuring they're thoroughly covered. Refrigerate for at least 1 hour or ideally overnight.

In a large bowl, mix shredded cabbage and julienned carrots until well combined.

In a small bowl, whisk together mayonnaise, lime juice, and sriracha if using. Combine with cabbage and carrot mixture.

Preheat a grill pan over medium-high heat. Grill marinated chicken thighs for 6-7 minutes per side until the internal temperature reaches 165°F (75°C).

Remove chicken from heat and let it rest on a cutting board for 5 minutes.

Lightly toast the brioche buns in the grill pan or toaster until golden brown.

Assemble the sandwich by layering sliced grilled chicken on the toasted buns, topping with the crispy slaw, fresh cilantro, and sesame seeds.