First make the sauce: Combine cornstarch and water in a small bowl. In a medium saucepan, add remaining ingredients along with the cornstarch/water mixture. Stirring frequently, bring sauce to a simmer. Cook until sauce lightly thickens. Sauce will be dark brown. Once sauce has thickened up, remove from heat and let cool completely. (place in fridge to speed up the process)
Once the sauce is completely cooled, place cubed chicken in a large ziplock bag. Pour half of the sauce over the chicken and gently shake to coat chicken. Let chicken marinate in the fridge for at least 30 minutes. Place remaining sauce aside for later.
When the chicken is done marinating, place your chicken on skewers and heat the grill to medium heat.
Once the grill is hot, place skewers on lightly oiled grill. Turn the skewers every 3-4 minutes until the chicken is cooked through (internal temp 165 degrees), about 20-30 minutes. During the last few turns baste with remaining teriyaki sauce.
Remove skewers from grill and serve with chopped green onion and a sprinkle of sesame seeds(optional).