Grease a 20cm square tin and line with baking paper.
Melt the butter in a small saucepan over a medium heat. Stir in the peanut butter.
Take the saucepan off the heat and sift in the icing sugar and cocoa. Add the vanilla extract and egg and mix well.
Break the biscuits into pieces and stir into the chocolate mixture. Tip the mixture into the prepared tin and spread into an even layer. Refrigerate until firm, then remove from the tin and cut into squares. Store in the fridge.