In a medium bowl, whisk together soy sauce, honey, gochujang, rice vinegar, sesame oil, garlic, ginger, and black pepper until well combined.
Place the chicken breasts in a resealable plastic bag or shallow dish and pour the marinade over the chicken. Seal the bag or cover the dish and refrigerate for at least 30 minutes, or up to 4 hours for more flavor.
Preheat your grill or a skillet over medium-high heat and add vegetable oil.
Remove the chicken from the marinade, allowing excess marinade to drip off. Discard the remaining marinade.
Grill or cook the chicken for about 6-7 minutes per side, or until the internal temperature reaches 165°F and the chicken is cooked through.
Remove the chicken from the heat and let it rest for 5 minutes before slicing.
Serve the chicken garnished with sliced green onions and sesame seeds.