Put the mix into a mould lined with baking paper and flatten out.
Mix the peanut butter with the sugar until smooth. Put tablespoon-sized dollops of it scattered around the brownie and make swirls using a chopstick.
Take to a preheated oven at 160C for 20-30min or until you can insert a toothpick at the center and it comes out with a bit of dough (we don't want it to dry out completely).