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 the sauce slightly thickens. Remove from heat and strain out the garlic and ginger pieces.
02 - Thread chicken cubes and spring onion pieces alternately onto the pre-soaked bamboo skewers, ensuring even distribution for consistent cooking.
03 - Heat a grill or grill pan over medium-high heat. Lightly oil the grates to prevent sticking during cooking.
04 - Place skewers on the grill and cook for 3 minutes per side. After turning, brush generously with the prepared yakitori sauce. Continue grilling, turning and basting frequently, for an additional 6-8 minutes until chicken is fully cooked and develops a glossy caramelized coating.
05 - Transfer skewers to a serving plate and drizzle with remaining yakitori sauce if desired. Serve immediately while hot.