Mix marinade ingredients; soy sauce, water, sugar and mirin. Add chcken and marinade in refrigerator for 30 minutes. Mix 4 teaspoons cornstarch with 2 tablespoons water and set aside. In small sauce pan mix together Teriyaki Sauce ingredients; soy sauce, water, sugar, Mirin, ginger and garlic powder. Bring to a boil. Add cornstarch mixture. Stir until thickened. Brush sauce on chicken and grill or roast (at 350 degrees)f until done (basting several times). Keep sauce warm while chicken cooks. Cut chicken in bite size pieces. Arrange on top of bowl of rice with additional teriyaki sauce and sprinkle with sesame seeds.