In a small bowl mix together the brown sugar, soy sauce, pepper flakes, vinegar, sesame oil, garlic, and ginger.
Pour approximately half of the mixture into a gallon-sized ziploc bag, add the chicken, squeeze out the excess air, and seal. Let the chicken marinate in the fridge for at least 15 minutes, and up to 1 hour.
Mix the remaining teriyaki marinade with 1 1/2 teaspoons of cornstarch and heat in a microwave safe bowl in 30 second increments, stirring between increments, until it thickens (alternatively, heat in a sauce pan while whisking).
Grill the chicken on a hot grill, or cook it on the stove with a cast iron grill pan until cooked through..
When you flip the chicken over, brush the top with the thickened teriyaki sauce.
The length of cooking will depend on the heat of your grill. Mine cooked for 5-7 minutes on the first side and another 5 minutes on the second side.