Prepare a 20cm brownie tray by lining with baking paper.
Pre heat the oven to 170 degrees C.
Melt butter and chocolate in a saucepan over a low heat. Stir until smooth. Remove from the heat and add the sugar and cocoa, stir until combined.
Stir through the flour, vanilla, baking powder then the eggs.
Once the batter is combined pour 3/4 of the mixture into the prepared tray.
Microwave the 1/2 cup of peanut butter until it's soft and liquid.
Pour the peanut butter over the brownie base.
Add the remaining brownie batter to the pan and swirl through gently.
Bake for approximately 45mins depending on your oven. Remove the brownie from the oven when the batter appears firm in the middle.
Cool brownies in the tray and cut when cool.