01 - In a mixing bowl, combine soy sauce, mirin, sake, honey, brown sugar, grated ginger, minced garlic, sesame oil, and rice vinegar, whisking until well blended.
02 - Place chicken thighs in a resealable bag or shallow dish. Pour marinade over the chicken, seal or cover, and refrigerate for at least 1 hour or up to overnight.
03 - Preheat grill or grill pan to medium-high heat.
04 - Remove chicken from the marinade, allowing excess to drip off. Reserve remaining marinade.
05 - Grill chicken for 6 to 8 minutes per side, or until charred at the edges and cooked through to an internal temperature of 165°F (75°C).
06 - While chicken grills, pour reserved marinade into a small saucepan. 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 in, cooking until thickened.
07 - Brush grilled chicken with the reduced sauce. Transfer to a platter, sprinkle with toasted sesame seeds and sliced scallions, and serve immediately.