Wash and pat dry the walnuts. Using a fork, prick the walnuts all over.
In a pot or pan, add the water and the salt and bring to a boil. Once it boils and the salt has dissolved, immediately turn off the heat and set aside to cool.
In a sterilised jar, pack the walnuts tightly (tight enough so they don't float up when adding the brine.)
Pour the cooled-down brine (salt-water) into the jar with the walnuts and seal tight. Leave the jar in a cool dark place for eight days.
After eight days, drain the brine. Place the walnuts onto a flat plate or tray, and leave them in the sun for a whole day. Turn the walnuts from time to time, so that they darken evenly. At the end of the day you will see that the walnuts have turned black.
Pour the malt vinegar and castor sugar into a pan and bring to a boil. Turn off the heat once the sugar has dissolved and set it aside to cool.
Once again, pack the walnuts into a sterilised jar tightly.
Pour the cooled-down pickling liquor into the jar with the walnuts and seal the jar shut. Set aside in a cool dark place (under 21ºC) for a month.
After a month, the pickled walnuts are ready to use as desired.