In a small saucepan over medium heat, whisk together the water, soy sauce, sugar, rice vinegar, sake, Hoisin, and ginger root. Simmer for about 10 minutes, or until slightly reduced.
While the mixture is simmering, mix the cornstarch and cold water together to form a slurry; add slowly to the simmering teriyaki sauce until it comes to your desired level of thickness. Remove from heat.
Use hot or at room temperature. Store, tightly covered, in the refrigerator for up to two weeks.
Notes
LindySez - this sauce will thicken as it cools so don't over thicken if you plan to store it. Also, bring to room temperature if refrigerated.