01 - Submerge bamboo skewers in water and soak for at least 30 minutes to prevent burning during grilling.
02 - Trim chicken thighs and cut into uniform bite-sized pieces, approximately 1¼ to 1½ inch squares, ensuring even cooking.
03 - Thread chicken pieces and spring onion segments alternately onto the soaked skewers, distributing evenly and leaving a small handle at the base.
04 - In a small saucepan, combine soy sauce, mirin, sake, sugar, minced garlic, and grated ginger. Bring to a boil over medium heat while stirring, then reduce to a simmer for 5 to 7 minutes until the sauce thickens slightly. Divide into two portions: one for basting and one for serving.
05 - Preheat a grill, broiler, or barbecue to medium-high heat, ensuring the grates are clean and lightly oiled.
06 - Place skewers on the hot grill and cook for 8 to 10 minutes, turning occasionally. Baste generously and frequently with the reserved yakitori sauce until the chicken develops a glossy, caramelized coating and is cooked through completely.
07 - Transfer yakitori to a serving plate and drizzle with the remaining reserved sauce. Serve immediately while hot.