In a medium bowl, whisk together soy sauce, mirin, sake, brown sugar, ginger, garlic, and black pepper to create the marinade.
Add the chicken pieces to the marinade, ensuring they are well coated. Cover and refrigerate for at least 30 minutes, or up to 2 hours for more flavor.
Preheat your grill or grill pan over medium-high heat.
Thread the marinated chicken pieces onto the soaked wooden skewers, leaving a little space between each piece.
Grill the skewers for about 5-7 minutes on each side, or until the chicken is cooked through and has nice grill marks.
Remove from the grill and let rest for a few minutes. Garnish with chopped green onions before serving.