Combine the water, soy sauce, vinegar and sugar in a large saucepan. Add the bean milk sheets and turn them into the sauce. Bring to the boil, then lower the heat and simmer for about 20 minutes until the sauce has been absorbed. Add the sesame oil.

Turn out on to a large piece of muslin and roll up the cloth into a sausage shape. Tie it up securely with lots of string. Steam it over hot water for 2 hours.

Remove from the heat and leave it to cool then chill it thoroughly before unwrapping and slicing it.