Add 3 tbsp turbinado sugar and 1 tsp dark brown sugar to a small saucepan and heat on medium.
Once it starts to melt, add 75 ml sake and 75 ml mirin. Stir occasionally while you bring it to a boil.
Allow to boil for 1-2 minutes and then turn the heat down to low, then add 75 ml Japanese soy sauce (koikuchi shoyu) and simmer for 10 minutes or until thickened.
Scoop out any foam that forms on the surface. Once thickened to a syrup-like consistency, remove from the heat and allow to cool.
Enjoy!
Notes
Store in the fridge in a sterilized jar for up to 2-3 months and use within 1-2 weeks of opening.