Line a 20cm square tin with non-stick baking paper.
Place the chocolate, cream and butter in a saucepan and stir over a low heat, stirring until mixture is melted and smooth. (I used the microwave for this step).
Place the biscuits in a mixing bowl and pour over 2/3 of the chocolate mixture and sitr to coat the biscuits. Spoon this into the prepared tin and pour over the remaining chocolate mixture. Tap the sides of the tin to remove any bubbles.
Refrigerate for 3 hours or until set. Remove from tin, dust with cocoa and cut into squares.