To make the crust: Stir the butter into the crushed biscuits, tip into the sandwich tin and press down well to cover the base and sides of the tin. Chill the tin for at least 30 minutes to allow the crust to set.
To make the filling: Pour the cream into a bowl and add the icing sugar and vanilla extract. Whisk until the mixture forms soft peaks, then gradually whisk in the melted chocolate until the mixture thickens, but taking care not to over-whip. Spoon the mixture into the biscuit case and spread out. Chill for a few hours, or overnight, until the filling has set. Dust with cocoa before serving.