Stir the sugar and both vinegars together in a small pot over a medium-low heat until the sugar dissolves.
Add all remaining ingredients and bring to the boil, then reduce the heat and simmer for 25 minutes or until it reduces to a sticky thick sauce.
TIP: if any foam forms, skim it away with a spoon and discard.
Notes
This recipe makes about ¾ cup of Amai sauce which is the perfect amount to dip 20 gyoza or potstickers. It can also be used as a sauce on Japanese curry or noodle dishes.Allow the sauce to cool before storing as hot liquid with sugar can cause serious burns if you're not careful.The sauce will thicken as it cools, so if you want a thinner consistency, simply add a little water when reheating.Serve amai sauce warm or cold.