Preheat the oven to 160°C (fan-assisted convection). Grease and line a 21 x 11cm loaf tin with baking paper.

Whisk the warm melted butter and sugar (sachet 1), then whisk in the eggs one at a time until well combined. Add the brownie mix (sachet 2) and stir to form a glossy batter.

Spread the batter evenly into the prepared tin. If your peanut butter is quite firm, gently soften it in the microwave in 10-second bursts. Dot on top of the brownie batter and use a skewer or sharp knife to swirl it into the surface.

Bake the swirled brownie for 20-25 minutes until cooked through but still fudgy in the middle. Allow to cool completely before cutting into squares.

Tips: For a higher yield, this recipe can also be baked in a 20cm square tin – reduce the bake time to about 15 minutes. Natural nut butters have a more fluid consistency compared to other varieties, so are ideal for swirling into your bakes. You can also swirl other toppings into your brownies, like Biscoff spread, Nutella, tahini or jam.