2 tbsp sugar (slightly more if you want it sweeter and more viscous)
2 tbsp mirin
2 tbsp soy sauce
Put all that into a saucepan and boil over a low flame. Once it bubbles, stir to dissolve all the sugar. Make sure your flame is not too big or the sauce will burn. Within minutes, you’ll see that the sauce will thicken slightly and turn glossy. Turn off the fire and leave it to cool.
Notes: Use very low heat! I burned the sugar the first time!
Used Japanese soy sauce. Added a hint of Chinese soy sauce