When it's done, put an Otoshibuta and a regular pot lid. The otoshibuta is to keep the soybeans under the cooking liquid (which is to prevent the beans from getting wrinkles). Reduce heat to low and simmer (make sure beans are not bouncing around) for 4 hours or until the beans are tender.
Check inside the pot a few times to make sure there is enough cooking liquid. If not enough, add water (I added 1 cup water). Skim the surface if needed.
Check if the beans are tender by mashing a bean with two fingers. Take out the rusty nails/Iron Fish.
When the beans are tender, add soy sauce and mix well.
Remove from the heat and place the parchment paper on top of the surface to prevent the beans from getting wrinkles. Once cooled, keep in the refrigerator overnight so the soybeans will turn darker and absorb more flavor.
To Serve and Store
The following day, you can transfer the soybeans to an airtight container and store in the refrigerator for up to 4 days or in the freezer for up to a month.
Serve the soybeans chilled or at room temperature in a bowl.
Leftover Cooking Liquid
You can use the leftover cooking liquid to make jello (using gelatin, agar, or kanten), latte, sweets, bread, oshiruko (zenzai), and more!