In a large bowl, whisk together soy sauce, brown sugar, honey, gochujang, rice vinegar, sesame oil, minced garlic, and grated ginger. Add the chicken thighs, coat well, cover, and marinate for at least 20 minutes at room temperature or up to 2 hours in the refrigerator.

In another bowl, combine shredded cabbage and julienned carrots. In a separate bowl, whisk mayonnaise, rice vinegar, sugar, salt, and pepper, then drizzle this over the cabbage mixture and toss. Cover and chill for at least 15 minutes.

Heat a skillet or grill pan over medium-high heat. Remove chicken from marinade, letting excess drip off, and cook for 6-7 minutes per side until internal temperature reaches 165°F (74°C). Let rest before slicing.

Butter each side of the buns and toast in a non-stick skillet until golden brown, about 2-3 minutes.

Assemble the sandwich by placing sliced chicken on the bottom bun, drizzling with extra marinade if desired, topping with cabbage slaw, and finishing with the top bun.