Grease a 20 x 10cm x 5cm baking tin with butter. Cut a piece of baking parchment to run along the middle and up the ends of the tin so that it hangs over the edges.
Break the biscuits into small pieces and put into a mixing bowl. Set aside.
Half fill a pan with water and bring to a simmer. Break the chocolate into small pieces and put in a heatproof bowl. Add the butter and the golden syrup, then place the bowl over the simmering water and leave for 2 minutes to melt. Stir for a further 1-2 minutes until the mixture is smooth and glossy. Remove from the heat and leave to cool slightly.
Pour the chocolate mixture in with the broken biscuits, along with the cranberries and pistachios. Stir to combine. Spoon the mixture into the tin, pushing it down with a spoon. Cover with cling film and refrigerate for 4 hours, or overnight.
When set, lift the cake out of the tin using the parchment strips – you may need to run a knife along the sides to release it from the tin. Cut into small squares to serve.