Soak the beans in about 3 times the amount of water for 1 to 2 nights. Change the water about twice a day.
When the beans are no longer wrinkled, they have been soaked for long enough.
Transfer the beans to a pot and add enough water to completely cover. Bring to a boil over high heat, then lower the temperature to medium and simmer for 5 minutes while skimming off the scum.
Drain into a sieve. Soak the beans in water again so that they don't dry out, and take off the skins. If any of the beans have sprouts, remove them too.
Once all the beans have been peeled, simmer in plenty of water until they are falling-apart tender. Bring to a boil over high heat first, then simmer over low heat, skimming off scum.
When they have fallen apart, if there is only a little water in the pan, add more and heat through. That makes the beans easier to pass through a sieve.
Scoop out a little at a time and pass the beans through a sieve. When everything has been processed, rinse out the pan.
Take the sieved beans and squeeze them tightly in a clean kitchen towel. Return the squeezed out bean paste to the pot. Add sugar and set the heat to low.
The sugar will melt and liquify. In about 20 minutes, the texture will change.
Keep an eye on the pot so that it doesn't burn. Cook until it's a bit looser than you want it to be, and turn off the heat. Add salt just before you turn the heat off and mix.
Transfer the bean paste to a storage container while it's still hot to prevent it from drying out, and smooth out the surface. It dries out easily.
I also have a recipe for ichigo daifuku (strawberry mochi dumplings). Recipe ID: 762283.
Story Behind this Recipe
I make ichigo daifuku a lot, but I live in the countryside, so it's hard to buy shiro-an. I tried making it in a microwave once, but I developed my own way, adjusting the sweetness to my taste.
Passing the bean paste through a sieve will result in a very smooth texture. If you transfer the bean paste quickly to a storage container, cover the surface with paper towels and close the lid, the surface won't get too dry or get too wet because of the condensation. If the lid of the storage container won't close properly, cover with plastic wrap.