01 - In a mixing bowl, whisk together soy sauce, mirin, brown sugar, honey, rice vinegar, garlic, ginger, and sesame oil until the sugar dissolves and the mixture is well combined.
02 - Transfer 1/4 of the marinade to a small saucepan and set aside for glazing. Pour the remaining marinade over the chicken pieces, tossing to coat evenly. Cover and refrigerate for at least 30 minutes, or up to 2 hours for deeper flavor penetration.
03 - If using wooden skewers, submerge them in water for at least 20 minutes to prevent burning during grilling.
04 - Remove chicken from the marinade, allowing excess to drip off. Thread the pieces onto skewers, leaving a small gap between each piece to ensure even cooking.
05 - Preheat a grill or grill pan over medium-high heat. Place the skewers on the grill and cook for 12 to 15 minutes, turning occasionally with tongs, until the chicken is cooked through and lightly charred on the edges.
06 - While the skewers grill, bring the reserved marinade in the saucepan to a boil over medium heat. Simmer for 2 to 3 minutes, stirring occasionally, until the sauce thickens to a glossy glaze consistency.
07 - Brush the grilled skewers generously with the thickened teriyaki glaze using a basting brush. Sprinkle with toasted sesame seeds and sliced spring onions. Serve immediately.