In a medium bowl, whisk together the soy sauce, mirin, sake, brown sugar, sesame oil, ginger, and garlic until the sugar is dissolved.
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 bamboo skewers, leaving a little space between each piece.
Place the skewers on the grill and cook for about 5-7 minutes on each side, or until the chicken is cooked through and has nice grill marks.
Remove the skewers from the grill and let them rest for a couple of minutes.
Garnish with green onion pieces before serving.