Soak wooden skewers in water for 10–20 minutes to prevent burning.

In a mixing bowl, combine soy sauce, a splash of water, red wine or mirin, brown sugar, and rice vinegar. Stir until the brown sugar dissolves.

Trim any excess fat from the boneless chicken and cut it into 1-2 inch chunks. Lightly pound the chicken pieces with a meat mallet.

Marinate the chicken with garlic, ginger, black pepper, and yakitori sauce for at least 10 minutes.

Pour remaining yakitori sauce into a saucepan, bring to a boil, then thicken with cornstarch slurry.

Thread marinated chicken onto soaked skewers. Preheat grill to medium-high heat. Grill for 8-12 minutes, turning and basting with sauce.

Once grilled, let skewers rest for a couple of minutes. Serve warm, drizzled with extra sauce.