Rinse the kuromame with plenty of water, and drain throughly.
Combine water, soy sauce, white sugar, brown sugar, salt, and baking soda in a pot and cook over high heat. Bring to a boil, turn off the heat, then add the beans from Step 1. Cover with a lid and let sit for about 5 hours.
Heat Step 2. Cook over high heat, then reduce to low heat once boiled. Skim the scum while simmering over low heat for about 4 hours. The simmering water from Step 2 must be reduced to a level that barely covers the beans. Make sure the beans are constantly submerged while simmering.
The beans are done if they become soft enough to be mashed between your fingers.
Do not remove the simmered beans from the pot right away, but let them gradually cool down (to prevent the skin of the beans from becoming wrinkled). Make sure the simmered water is kept over the beans when storing.
Right after simmering, the beans will taste a bland and not flavorful enough, but they taste just right in two days.
I added some edible gold leaf since it's New Year's.
Story Behind this Recipe
My mother's recipe, to which I used to refer, was less satisfying because it called for a certain amount of water but actually required the occasional addition of water. So, to avoid that extra step, I increased the amount of water considerably. Blending brown sugar into the white sugar will give it a light and yet rich taste.
My recipe doesn't call for additional water later in the cooking process, but if the simmering water level becomes lower than the beans, then add water. Old beans -- harvested more than six months before use -- may take up to twice as long to become tender.