01 - Combine soy sauce, mirin, sake, sugar, garlic, and ginger in a small saucepan. Bring to a boil over medium heat, then reduce heat and simmer for 8–10 minutes until slightly thickened. Remove from heat and discard garlic and ginger slices.
02 - Thread chicken cubes and spring onion pieces alternately onto the pre-soaked bamboo skewers, leaving space at both ends for easy handling.
03 - Preheat grill or grill pan over medium-high heat. Lightly oil the grates to prevent sticking.
04 - Place skewers on the grill and cook for 3 minutes on each side, brushing 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 - Transfer skewers to a serving platter and drizzle with remaining yakitori sauce if desired. Serve immediately while hot.