Prepare the Marinade. In a mixing bowl, whisk together soy sauce, brown sugar, garlic, ginger, rice vinegar, and sesame oil until combined. Reserve half of the mixture for basting and glaze.
Marinate the Chicken. Place the chicken in a large zip-top bag or shallow dish. Pour the remaining marinade over the chicken, ensuring it’s fully coated. Seal and refrigerate for 30 minutes to 8 hours.
Preheat the Grill. Preheat your grill to medium-high heat (about 375–400°F). Lightly oil the grates to prevent sticking.
Grill the Chicken. Remove the chicken from the marinade, letting any excess drip off. Grill the chicken for 5–7 minutes per side, basting with the reserved marinade during the last few minutes of cooking. Use a meat thermometer to ensure the internal temperature reaches 165°F.
Thicken the Glaze. While the chicken grills, bring the reserved marinade to a boil in a small saucepan. Stir in a cornstarch slurry (1 tablespoon cornstarch mixed with 1 tablespoon water) and simmer until thickened.
Serve and Garnish. Transfer the grilled chicken to a serving platter. Drizzle with the thickened teriyaki glaze and garnish with sesame seeds and chopped green onions.