Cut the boneless skinless chicken thighs into 1-inch pieces and set aside.

In a separate bowl, whisk together soy sauce, mirin, sake, brown sugar, ginger, garlic, and black pepper until the sugar dissolves.

Coat the chicken pieces in the marinade and refrigerate for at least 30 minutes.

Preheat your grill to medium-high heat (about 400°F).

Thread the marinated chicken pieces onto the soaked wooden skewers, optionally adding green onions or veggies.

Grill the skewers for about 5-7 minutes on each side until cooked through, reaching an internal temperature of 165°F.

Brush skewers with remaining marinade during the last few minutes of grilling.

Let the skewers rest for a few minutes before serving, garnished with green onions and accompanied by rice or salad.