In a large bowl, mix the crushed biscuits with ⅔ of the melted butter. Press into the base and up the sides of a 20cm round, loose-bottomed cake tin.
Gently warm the cream and remaining butter in a pan over a low heat until just below boiling point. Remove from the heat and add the chocolate, stirring until melted.
Allow the mixture to cool for 5 minutes, then pour into the base. Sprinkle over the pistachios and chill for 1-2 hours until set.