In a medium bowl, whisk together soy sauce, mirin, sake, brown sugar, sesame oil, ground ginger, and black pepper.
Add the chicken pieces and toss to coat. Cover and marinate in the refrigerator 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 skewers, alternating with pieces of green onion.
Place the skewers on the grill and cook for about 5-7 minutes per side, or until the chicken is cooked through and has nice grill marks.
Brush with any remaining marinade during the last few minutes of cooking for added flavor.
Remove from the grill and let rest for a few minutes before serving. Garnish with additional green onions if desired.