I recommend this for kuromame novices! They're made in a rice cooker and get so soft! It's safe to just leave it alone and has such an simple, home-made taste.
Note: the iron oxide in the rusty nails reacts with the tannins in the beans, and thus naturally darkens in color and gives a shiny finish. The same effect will happen if the beans are cooked in an iron skillet.
Put the all the ingredients except for the kuro-mame beans into a pot and boil. Transfer to the rice cooker.
Add the kuromame beans into the rice cooker and let sit overnight.
The next morning, cook on "okayu" (rice porridge) mode twice, then let it sit. It will be ready to eat that night.
Story Behind this Recipe
I wanted to try making soft black beans even though I didn't have a pressure cooker. Cooking it on normal mode ended in a disaster, but it's perfect on okayu (rice porridge) mode! They're so soft, and crumble on your tongue!! This recipe can be made in your rice cooker.
Be sure to set your rice cooker on okayu (rice porridge) mode. If you do it on normal mode, you're going to have a bad time. (I already tried this, so please don't make my same mistake! I guess there may be some rice cookers that this is not possible to cook this in, so the first time you try it, keep an eye on it. Before going to bed, I set it to okayu mode, then once again in the morning. Then, when I come home that night, they're ready to eat.