Grease a 500 g (1 lb) loaf tin.
Heat the butter, evaporated milk, caster sugar, measurement water and vanilla extract gently together in a heavy-based pan, stirring until the sugar has dissolved. Bring to the boil and boil the mixture for 10 minutes, stirring all the time. Test to see if it is ready by carefully dropping ½ teaspoon of the mixture into some cold water – it should form a soft ball. Pour half the fudge mixture quickly into a heatproof jug. Add the chocolate to the remaining fudge mixture in the pan and stir to melt.
Pour half the chocolate fudge into the base of the loaf tin, then carefully scatter over half the biscuits. Pour the vanilla fudge on top and scatter over the remainder of the biscuits. Finish with a final layer of chocolate fudge. Cool, cover with clingfilm and chill overnight.
Turn out the fudge onto a board and cut into pieces.