Set the oven to 170 C / Gas 3 and grease a 20x20cm square cake tin.
Sift the flour and baking powder.
In another bowl, cream the butter and peanut butter until soft. Add the sugar, egg and vanilla extract, then beat. Add the flour mix and the chocolate; then mix to form dough. Place the mixture into the tin.
Bake for 25 to 30 minutes. When taken out, wait until cool and then slice and remove from tin. Hope you enjoy! ;)