This yakitori sauce is so versatile and can keep refrigerated almost indefinitely. Not only great for dipping in skewers to be cooked over a grill, you can also use this as a glaze for pan roasted meats such as pork chops.
Bring the water to a boil in a non-reactive sauce pot, then add the skin and cook for 10 to 15 minutes. Do not skim the foam (aku) that forms on the top - usually it gets skimmed, but in this case does not. Remove and discard the skin. Add the soy, sake and mirin, and reduce by one third. Strain through a cheese cloth-lined strainer, chill and refrigerate if not using right away.