In a small bowl whisk together soy sauce, brown sugar, rice vinegar, water, garlic and ginger. Pour half of the mixture over chicken and let marinade for at least 2 hours preferable over night.
Meanwhile place the remaining marinade in a small pot and place over medium heat. Add the cornstarch and mix until dissolved. Bring to a boil, reduce heat and simmer 3-4 minutes. Remove from heat and set aside.
Heat a grill or cast iron pan over high heat and cook 6-7 minutes on each side to get a nice sear. Brush the teriyaki sauce on. Cook for another 3-4 minutes on each side or until juices run clear.
Let chicken rest 10 minutes. Serve with rice and top with green onions and sesame seeds