Grease an 18cm x 28cm rectangular slice pan and line base and sides with baking paper, extending paper 2cm above edge of pan.
Place Arnott’s Caramel Crowns biscuits into the bowl of a food processor and process until the mixture resembles fine crumbs. Add butter and peanut butter, process until just combined.
Press the mixture over the base and side of the prepared pan. Refrigerate until set.
In the meantime, place chocolate and butter in a medium microwave safe bowl. Place in the microwave for 20 second bursts on medium power until melted, stirring in between each burst. Pour chocolate over slice.