Add the soy sauce, mirin and sugar to a small pot and bring it to a simmer.
Reduce the heat to low and simmer for about 15 minutes, or until the mixture is reduced to about half. Stir occasionally. You will know the sauce is done cooking when you run a spoon through it and it leaves a streak.
Remove the pot from the heat and stir in the butter.
Brush the glaze onto cooked corn on the cob and serve immediately. Enjoy!