Combine the soy sauce, brown sugar, cider vinegar, ginger, and garlic powder in a small saucepan over medium heat. Cook for 5 minutes to reduce slightly.
Pour half the mixture into a bowl large enough to marinate the chicken.
Mix the cornstarch with water in a bowl and add it to the remaining sauce. Simmer for about 3 minutes until thickened. Set aside.
Place the chicken in the marinade bowl. Place it in the refrigerator to marinate for 2 hours.
Heat the grill to 400° F(205°C).
Once the grill is hot, place chicken on the grill. Turn every 5 minutes until the chicken is cooked to an internal temp of 165°F(75°C), about 15-20 minutes. During the last few turns baste with teriyaki sauce.
Remove the chicken from the grill. Garnish with sesame seeds and serve.