01 - Combine soy sauce, mirin, sake, sugar, garlic, and ginger in a small saucepan. Bring to a boil over medium heat, then simmer for 8-10 minutes until slightly thickened. Remove from heat and discard garlic and ginger pieces.
02 - Thread chicken and spring onion pieces alternately onto the pre-soaked bamboo skewers, ensuring even distribution for consistent cooking.
03 - Preheat grill or grill pan over medium-high heat. Lightly oil the grates to prevent sticking.
04 - Grill skewers for 3 minutes on each side, brushing generously with yakitori sauce after each turn. Continue grilling, turning and basting frequently, for another 6-8 minutes until chicken is cooked through and develops a glossy, caramelized coating.
05 - Serve immediately while hot, drizzling with any remaining yakitori sauce if desired.