Another seasoning that you can easily make with koji is Shoyu Koji (soy sauce koji). We posted how to make shio koji before but shoyu koji is even easier.
Koji is type of bacteria that is used in many japanese seasonings including soy sauce, miso and beverages such as sake and amazake. After fermenting with koji the flavour deepens and real UMAMI flavour comes through.
Getting Koji may be a little tricky especially if you are looking for organic. I get mine sent from japan and also from a local organic miso maker who spares his with me. You may be able to find some from a relatively large Japanese grocery store.
All you need to make Shoyu Koji are
soy sauce / tamari for gluten free option
Combine about the same amount of each and ferment for 1-4 weeks. ( Depending on the temperature outside. warmer it is faster it ferments)
Make sure to stir every day and if it seems dried up you can add an extra soy sauce or tamari.
I have also explained a difference between soy sauce (shoyu) and tamari in the video