Cut the chicken thighs into 1-inch pieces and set aside.
In a bowl, whisk together the soy sauce, mirin, sake, brown sugar, ginger, garlic, and sesame oil 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 bamboo skewers, alternating with pieces of green onion if desired.
Place the skewers on the grill and cook for about 4-5 minutes on each side, or until the chicken is cooked through and has nice grill marks.
Brush with additional marinade during the last few minutes of cooking for extra flavor.
Remove the skewers from the grill and let them rest for a couple of minutes before serving.
Enjoy your grilled chicken yakitori warm, garnished with extra green onions if desired.