Mix80g (2¾oz) white miso with 1tablespoonmirin. Spread half of the miso mixture in a container and make two dents with the back of a rolling pin. Lay a muslin cloth over the miso and put 2egg yolks in the dents. Lay another piece of muslin on top. Cover the yolks with the remaining miso.
Cover the container and keep in the fridge for 4 days. The yolks should be firm with a jelly-like texture and a vibrant orange colour.