Cook the soaked beans in 4 cups water until completely soft and collapsing (45–60 minutes).
Reserve 1 cup of the cooking liquid. Strain the beans and mash them coarsely.
Return the beans to the pot with the reserved liquid, sugar and mirin. Cook over low heat, stirring and mashing constantly, 30–40 minutes until it pulls away from the pot in a thick paste.
Press through a sieve for a silky paste (or leave textured). Cool and store wrapped in the fridge for up to 2 weeks.
