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