Gently slit the skins of the chestnuts with a sharp knife, but be careful not to cut into the nuts. Boil the nuts for 20 minutes, then skin them while still warm. Make a syrup from the sugar and water, stirring to dissolve the sugar and boiling them together. Add the vanilla essence and chestnuts. Boil briskly for 10 minutes, then take the nuts out and drain them on a wire rack. Leave for 24 hours, then re-boil the syrup, put the nuts back and simmer until they are thickly coated. Drain as before, and allow to dry before putting into airtight containers.