The night before, put all ingredients marked ☆ in a pot and bring to a boil. Add the rinsed beans, turn the heat off, and leave overnight with the lid on.
The next morning, bring to a boil over high heat. Please watch it or it might boil over.
When the pot comes to a boil, turn the heat down to very low. Simmer for 3 hours with the lid on.
Gently squeeze a bean. When the beans have become as soft as you want them to be, turn the heat off. Leave the beans as-is without disturbing them until they have cooled down.
Kintoki-beans seem to cook faster than kuromame. Check the beans for tenderness occasionally.
Story Behind this Recipe
My father-in-law grows kuromame, so I make this all year round, not only for New Years. After a lot of trial and error, I can say that this method is foolproof. I've made them not that sweet so that they can be enjoyed guilt-free. I heard if you simmer the beans with a rusty nail or a piece of iron, the color turns beautifully black.
Simmer with the lid on so the beans aren't exposed to the air. After simmering, keep the beans immersed in the cooking liquid until they have cooled down. You can avoid any boil overs if you fill a big pot about 60% of the way. If you heat the beans through once a week, you can keep them for several weeks in the refrigerator. When you re-heat the beans some scum will rise to the top, so skim that off.