Step 1

Preheat oven to 180°C (160°C fan) mark Grease and line a 20.5cm (8in) square tin with baking parchment. To make the brownie, melt butter and chocolate (stirring frequently) in a large pan until melted and smooth. Take off the heat and stir in sugar, then the eggs. Sift over the flour and cocoa powder and mix to combine. Set aside.

Step 2

In a medium bowl, mix together the ingredients for the peanut butter marble.

Step 3

Spoon about one-third of the chocolate mixture into the tin and spread to level. Dollop over a few spoonfuls of peanut mixture. Top with remaining chocolate mixture and spread to level. Drop more peanut spoonfuls on top, then tap the tin down firmly on a surface to release any air bubbles. Finally, run a knife or skewer through the mixture to marble it.