01 - In a mixing bowl, whisk together soy sauce, mirin, sake, honey, brown sugar, grated ginger, garlic, sesame oil, and rice vinegar until well combined.
02 - Place chicken thighs in a resealable plastic bag or shallow dish. Pour the marinade over the chicken, seal or cover tightly, and refrigerate for at least 1 hour or up to overnight for maximum flavor penetration.
03 - Preheat your grill or grill pan to medium-high heat, approximately 400°F.
04 - Remove chicken from the marinade, allowing excess to drip off. Reserve the remaining marinade for basting. Grill chicken thighs for 6 to 8 minutes per side until charred on the edges and fully cooked through, reaching an internal temperature of 165°F.
05 - While the chicken grills, pour the reserved marinade into a small saucepan. Bring to a boil over medium heat, then simmer for 2 to 3 minutes. If a thicker consistency is desired, dissolve cornstarch in 1 tablespoon of cold water and stir into the simmering marinade, cooking until thickened.
06 - Brush the grilled chicken generously with the reduced sauce. Transfer to a serving platter, sprinkle with toasted sesame seeds and sliced scallions, and serve immediately.