In a small saucepan, combine the soy sauce, mirin, sake, sugar, minced garlic, and minced ginger.
Bring to a boil over medium heat, then reduce the heat and let it simmer for about 10 minutes until the sauce thickens slightly. Remove from heat and set aside.
Season the chicken pieces with salt and pepper.
Thread the chicken pieces and green onion pieces alternately onto the soaked wooden skewers.
Preheat your grill or grill pan over medium-high heat.
Lightly brush the skewers with vegetable oil.
Place the skewers on the grill and cook for about 3-4 minutes on each side, or until the chicken is cooked through and slightly charred.
During the last few minutes of grilling, brush the chicken skewers with the prepared sauce (tare) and continue to cook, turning and basting frequently, until the chicken is glazed and caramelized.
Remove the yakitori from the grill and brush with any remaining sauce.
Enjoy your meal!