Preheat the oven to 180 C/ 350F. Mix the butter, sugar, and water until combined. Add the flour and mix. Wrap some of the dough mixture around each half block of chocolate. This is a fairly tricky task and would be helpful if you got others to help.
Bake on a lined baking tray for 20 minutes. They don't need to go too golden. Roll in caster sugar and allow to cool.
Notes
Store leftover biscuits in a cookie jar or sealed container. They will not last long.