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)

Melt butter in large pan over low heat.

Snap chocolate into squares and throw into a small bowl. Melt in the microwave in 30 second blasts, stirring well between each addition or sir the bowl over a pan over simmering water. Make sure that the bottom of the pan does not touch the water or the chocolate mat seize and go really grainy.

Blitz the digestive biscuits and brown sugar in a blender or food processor to give fine crumbs. Tip them into the melted butte. Stir the peanut butter and vanilla extract and mic together so everything is well combined.

Tip the mixture into the lines tin and press it down really hard with the back of the spoon. It needs to be really compact and tight. Then pour over melted chocolate, tilting the tin back and fourth a bit so that the whole thing is covered. Pop in the freezer for 30 mins to firm up (or the fridge for an hour)

Once the chocolate is set, remove it from freezer (or fridge). Lift it 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 and serve.