Soak the raisins in the cognac. Pour the milk in a small pot and put on the stove. Once it heats up, add the butter to it and stir until it melts.
Next, add the crumbled chocolate and stir until fully dissolved. Add the vanilla and raisins with cognac.
Stir the mixture, leave to cool. Crumble the biscuits into small pieces, add them to the chocolate mixture. Stir everything and form into a roll onto foil.