Line a 20cm/8in square tin with baking parchment, leaving some excess paper hanging over the edges (this makes it easier to lift out once set).Blitz the digestive biscuits and brown sugar in a blender or food processor to give fine crumbs. Add the melted butter, peanut butter, vanilla essence and toasted peanuts and mix together until everything is well combined. Tip the mixture into the lined tin and press it down really hard with the back of the spoon. It needs to be really compact and tight. Next, pour over the melted chocolate and even out with a palette knife. Place in the fridge for an hour to set. Once the chocolate is set, lift out of the tin with the help of the baking parchment. Remove the paper and then use a sharp knife to divide it into 16 squares before serving.