01 - In a medium mixing bowl, combine soy sauce, mirin, sake, honey, brown sugar, ginger, garlic, sesame oil, and rice vinegar. Whisk thoroughly until the marinade is fully blended.
02 - Place the chicken thighs in a resealable plastic bag or shallow dish. Pour the marinade over the chicken, seal or cover, and refrigerate for at least 1 hour or overnight for deeper flavor infusion.
03 - Preheat the grill or grill pan to medium-high heat.
04 - Lift chicken thighs from the marinade and allow the excess to drip away. Reserve the remaining marinade for basting sauce.
05 - Place chicken thighs on the grill. Cook for 6 to 8 minutes per side until edges are charred and internal temperature reaches 165°F.
06 - Meanwhile, pour reserved marinade into a small saucepan and bring to a boil over medium heat. Simmer for 2 to 3 minutes. For a thicker sauce, dissolve cornstarch in 1 tablespoon water and stir it into the marinade, cooking until thickened.
07 - Brush the grilled chicken with the reduced sauce. Arrange on a platter, sprinkle with toasted sesame seeds and sliced scallions, and serve immediately.