1.Preheat the oven to 160°C (140°C fan-forced). Grease a 20cm x 30cm lamington pan; line the base and two opposite sides with baking paper.
2.In a medium saucepan, combine butter and chocolate; stir over low heat until smooth. Transfer to a large bowl. Stir in sugar, eggs and essence, then sifted flours, cocoa and extra chocolate.
3.Pour mixture into the prepared pan. Drop small spoons of peanut butter into the chocolate mixture and swirl through the chocolate with a knife.
4.Bake for about 50 minutes or until firm. Allow to cool in pan.
This recipe can be made a week ahead.Note