Preheat oven to 175°C then grease and line a 20cm square pan with baking paper.
Sift together the plain flour, cocoa powder, and salt. Set aside.
Place butter and chocolate in a bowl over a pot of simmering water. Allow to melt then set aside to cool slightly.
Combine eggs and sugar in a bowl and beat with an electric mixer until light and fluffy. Add the chocolate mixture in a slow and steady stream then stir through the vanilla.
Fold through the dry ingredients being careful not to over mix then pour the batter into the prepared pan. Dollop peanut butter over the batter and swirl with a skewer.
Bake brownie for 20 minutes then remove from oven and drop pan down on your benchtop a couple times (this gives the brownie a dense and not cakey texture). Place it back in the oven to bake for another 20 minutes. Let the brownies cool completely in the pan before removing and portioning.