Preheat the oven to 350f/180c. Line a square baking tin with parchment paper. Use two long strips in a + shape so that you have handles to lift the brownies out later!
Add the chocolate and butter to a microwave safe mixing bowl and heat until melted. Alternatively, melt in a bain marie.
Once the chocolate and butter are melted, add in the sugar and whisk well for 1-2 minutes.
Whisk in the eggs and the vanilla.
Fold in the cocoa powder, flour and salt. Mix well, the batter should be very thick.
Spread the batter in to the prepared pan, using a spatula or spoon to spread it in to the corners.
Dot the peanut butter on top of the brownies and use a spoon or knife to swirl through the peanut butter and brownie mixture in figures of 8 until marbled.
Bake for roughly 25 minutes. To check if they are done, jiggle the pan, if the centre wobbles, give them a few more minutes.
Remove from the oven and allow to cool completely in the pan before lifting out and cutting in to 16 squares.