Melt the butter in a saucepan or in the microwave in a microwave-safe bowl. Mix in the sugar, eggs and vanilla. Then, mix in the cocoa powder, all-purpose flour, salt, and baking powder. Fold in the chocolate chips. Pour 1/2 of the batter into the pan, spreading it out on the bottom as best as you can. Then pour the melted peanut butter on top of the batter, spreading it around in an even layer. Spread the remaining half of the batter on top. It is ok if there is peanut butter coming through to the top!