Line a 20cm square tin with baking parchment leaving excess paper over the edges.
Melt the butter in a large pan over a low heat.
Melt the chocolate squares in a microwave or over a pan of simmering water.
Blitz the biscuits and brown sugar in a processor to create fine crumbs.
Stir the crumb mixture, peanut butter, and vanilla into the melted butter.
Press the mixture firmly into the tin and spread the melted chocolate over the top.
Chill in the freezer for 30 minutes or the fridge for 60 minutes until firm.
Lift out of the tin and cut into 16 squares with a sharp knife.