-
1
Place the sorted beans and the iron nail or iron peg in a large bowl. Cover with cold water (roughly 1 litre) and leave to soak overnight — a minimum of 12 hours. The iron begins leaching ions into the liquid from the outset; the soaking water will take on a dark, slightly blue-tinged colour. The beans should be visibly swollen and smooth-skinned before proceeding.
-
2
Transfer the beans, the iron piece, and all of the soaking liquid into a heavy pot. Bring to a gentle heat. As the liquid approaches a simmer, skim any grey foam that rises. Reduce the heat immediately to the lowest possible setting — the surface should show only a bare, occasional bubble. This is where the dish lives or dies: a rolling boil will tear the softening bean skins before the sugar has time to set their structure, producing permanent wrinkling and broken skins that cannot be repaired.
-
3
After approximately 30–40 minutes of bare simmering, add the first third of the sugar (approximately 50 g) and stir gently to dissolve. The beans should be beginning to soften but will not yet yield to pressure. Continue simmering.
-
4
After a further 30–40 minutes, add the second third of the sugar (approximately 50 g). Stir gently. The liquid will have reduced somewhat; add a little hot water if the beans are no longer fully submerged. The beans must remain covered throughout.
-
5
After a further 30–40 minutes, add the final third of the sugar (approximately 50 g). Stir gently and continue to simmer. The syrup will gradually thicken and take on a deep, glossy black sheen. The beans are done when a bean pressed between the fingers yields completely to gentle pressure with no hardness remaining at the centre.
-
6
Remove from the heat. Leave the beans to cool entirely in the cooking liquid, fully submerged and covered with a lid or drop lid (otoshibuta). Surface exposure during cooling causes colour loss and skin wrinkling. The finished beans, once cooled, should be plump, smooth-skinned, shiny, and a vivid, stable deep blue-black.