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 throughout.
03 - Thread chicken and spring onion pieces alternately onto the soaked skewers, distributing evenly and leaving a small handle at the bottom.
04 - In a small saucepan, combine soy sauce, mirin, sake, sugar, minced garlic, and grated ginger. Bring to a boil over medium heat, stirring to dissolve the sugar. Reduce heat and simmer for 5 to 7 minutes until the sauce thickens slightly. Remove from heat and divide into two portions — one for basting during grilling and one reserved for serving.
05 - Preheat an outdoor 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 to ensure even char. Baste frequently with the reserved yakitori sauce, building a glossy, caramelized glaze on the chicken. Continue grilling until the chicken is fully cooked through and reaches an internal temperature of 165°F.
07 - Transfer the grilled yakitori to a serving platter. Drizzle with the remaining reserved sauce and serve immediately while hot.