Preheat oven to 170 degrees Celsius (fan forced). Line a square cake tin (or a square 8 x 8 pan; which I didn't have) with baking paper.
Mix the flour, cocoa powder, baking powder and salt in a bowl. Set aside.
In a large mixing bowl, beat the sugar into the melted butter. Beat in the eggs one at a time. Stir in the vanilla extract and combine thoroughly. Add the flour mixture and combine well.
In a separate microwaveable bowl, pop the peanut butter and Nutella into a microwave and heat it up for 10-15 seconds (or until runny). Fold this into the brownie mix. I didn't mix this thoroughly as I wanted to keep the peanut butter swirls intact upon baking.
Pour the batter into the lined baking tin and bake for 45-55 minutes (check with the skewer test) in the oven.
Let cool in the tin for 2-3 minutes before removing. Cut into as many or as little pieces as you require and serve warm with a glass of milk.