01 - In a mixing bowl, combine soy sauce, mirin, sake, honey, brown sugar, grated ginger, minced garlic, sesame oil, and rice vinegar, whisking to form a marinade.
02 - Place chicken thighs in a resealable plastic bag or shallow dish, pour the marinade over, seal or cover, and refrigerate for at least 1 hour or up to overnight.
03 - Set grill to medium-high heat and allow to preheat fully.
04 - Remove chicken from marinade, letting excess drip off; set aside remaining marinade for sauce.
05 - Grill chicken thighs for 6 to 8 minutes per side, or until charred and internal temperature reads 165°F.
06 - Meanwhile, transfer reserved marinade to a small saucepan. Bring to a boil over medium heat and simmer for 2 to 3 minutes. For a thicker sauce, mix cornstarch with 1 tablespoon water and whisk into the sauce, cooking until thickened.
07 - Brush grilled chicken thighs with the reduced sauce, arrange on a platter, and top with toasted sesame seeds and sliced scallions. Serve immediately.