In a liquid measuring cup or small bowl combine the hot water and the brown sugar. Stir to dissolve the sugar. Stir in the remaining marinade ingredients.
Place the chicken thighs in a large zip top bag. Pour in the homemade teriyaki marinade and toss to coat the chicken. Seal the bag and place in the refrigerator to marinate for at least an hour and up to 48 hours. Alternatively you can use a large bowl and cover with a lid or plastic wrap.
When ready to cook, preheat the grill to high heat (500F). Remove the chicken from the marinade and reserve the excess marinade to make a glaze for the chicken. Place the chicken on the grill grates over direct heat and cook for 4-5 minutes, until grill marks form. Flip to the other side and cook for a few minutes more or until the internal temperature of the chicken registers at least 165F. Thighs are more forgiving than boneless skinless chicken breast and are better when cooked until 185F.
To make a glaze: Pour the reserved marinade remaining in the bowl into a small saucepan and heat over medium-high heat. Let simmer for 5-8 minutes or until it thickens. For a thicker sauce, you can combine 1 tablespoon of cornstarch in 2 tablespoons of water to make a cornstarch slurry. Stir into the simmering sauce to thicken. Spoon the homemade sauce over cooked chicken just before serving. Garnish with sesame seeds and sliced green onions if desired. (note: any uncooked marinade must be discarded)
Serve chicken with a sprinkle of sesame seeds and sliced green onions, if desired. Chicken teriyaki is also delicious served with grilled fresh pineapple.