Pour the mirin and sugar into a saucepan. Bring to a boil and cook until the liquid reduces by one-third.
100 ml mirin, 125 g sugar
Add the dashi powder and soy sauce, then bring to a boil and cook for 3 minutes. Dissolve about one tablespoon of kuzu in a little cold water, then stir it into the sauce.
Continue stirring for another 3 minutes, until the sauce is glossy and slightly thick.
Notes
To check if the sauce has reduced enough in the first step, pour a small amount of the mixture into cold water. If it forms a soft ball as it cools, the reduction is sufficient.You can substitute potato starch or cornstarch for kuzu, but the texture won't be quite the same.