1 Grease and line a small but deep square tin and break the chocolate into roughly similar sized pieces.
2 In a pan heat the cream and butter together. Bring to the boil then remove from the heat immediately. Allow to sit for 5 minutes then add the chocolate. Do not stir and allow to sit for a further 5 minutes.
3 This process prevents the mixture from seizing or splitting. Once the time is up you can stir the mixture. It should be smooth and glossy and almost entirely melted.
4 Stir in the rum if using and the vanilla. Once smooth transfer into your prepared tin and level off. Allow to set in the fridge for at least two hours. Cut the truffle mix into small squares and roll each piece in the chopped pistachios. Store in the fridge until required.