Rinse the soybeans several times.
Put the soybeans in a large container and cover with water 10 cm above the beans. Soak for 18 hours.
Drain the beans.
Put the beans in a pressure cooker and add enough water to cover. Cook for 5 minutes and let the pressure out.
Mix the salt and rice koji in a large bowl.
Once the soybeans have finished cooking, mash them with a potato masher or in a food processor. If it looks a little dry, add a splash of water.
Leave to cool till room temperature or slightly warmer.
Add the soybeans to the salt and koji. Make sure the soybeans have cooled as if they are hot, it will kill the mold in the koji.
Once fully mixed together, roll them into tennis-sized balls.
Slam the balls into a large glass container and push down to remove all air pockets.
Repeat until all the balls are in the container. Wipe the exposed inner sides of the container with an alcohol wipe to remove any lingering bacteria.
Sprinkle 1 tbsp of salt on top and cover with plastic wrap touching the surface of the miso.
Place heavy objects (e.g rock salt) on top to keep pressure on the miso.
Store the miso in a cool, dark place for at least 3 months.
Every 2-3 weeks, stir the miso from bottom to top to avoid any mold growing.
Divide the miso into different containers and use for any dish you like!