Combine ginger, garlic, half of scallion, soy sauce, sake, hoisin sauce and tea.
Reserve at least half of this for your sauce, adding the brown sugar to it. Use other half to marinate chicken for at least an hour, in a ziploc bag in the frig (this cuts down on the amount of marinade needed).
Remove chicken and barbecue (or bake) until cooked through.
Pour reserved sauce into a small pot and bring to a boil. Add cornstarch and water mixture and cook for a couple minutes, until slightly thickened. Serve chicken topped with reserved scallions. I paired this with brown rice and cucumber salad, but any veggie would probably go great with it.