Place the biscuits and butter in a food processor and process until it resembles fine breadcrumbs. Press the mixture into a 32 x 7½cm rectangular fluted loose-bottomed tart tin lined with non-stick baking paper. Smooth over with the back of a spoon. Place the chocolate and cream in a small saucepan over low heat and stir until the chocolate is melted and smooth. Stir in the nuts. Pour the chocolate mixture over the base and smooth the top. Place the slice in the fridge for 2 hours or until firm to the touch. Carefully remove from the tin and slice to serve. Serves 6−8.