Heat the milk, butter, sugar, vanilla and water. Let the mixture boil for 10 min., while stirring nonstop.
Add the chocolate and stir until it melts. Smear a baking form with the oil.
Pour half of the mixture into the form, sprinkle with half the crumbled biscuits, pour in more of the chocolate mixture, then biscuits and finish with the chocolate mixture.
Cool in the fridge overnight, cut it into pieces and serve.