Grease and line a 20cm square tin. Preheat oven to 180ºC/160ºFan/Gas 4
Melt the peanut butter in a small saucepan and set aside.
Add the chocolate and butter to a small pan and melt over a very gentle heat and set aside.
Add eggs, white sugar, brown sugar and vanilla extract to a mixing bowl and whisk together.
Add the melted chocolate and butter mixture and whisk until smooth.
Add the flour and mix until combined.
Add half the brownie mix to the tin and freeze for 5 minutes.
Pour the melted peanut butter over the brownie mix and top with the remaining brownie mix.
Bake in the oven for 35-45 mins until a skewer inserted comes out fudgy. Once cooked, the mixture will be dry and cracked on top, soft to the touch in the middle but won't wobble if you shake the tin.
Leave the brownie to cool completely in the tin before you turn it out and slice it.