Place the butter and peanut butter into a microwave-safe bowl. Heat for 1 minute to melt. Add the caster sugar and sifted cocoa powder and stir. Heat in the microwave on 30-second bursts (50% power) until the sugar has dissolved and ingredients combined - stirring each time.
Add the vanilla extract and the eggs and whisk until combined.
Sift the plain flour into the bowl and gently fold through.
Pour the peanut butter brownie mixture into the prepared tin. Dollop the extra peanut butter over the top of the brownie. Use a knife or a skewer to swirl the peanut butter through the brownie batter. Bake for 25-30 minutes or until cooked through.