Heat the oven to 200C/180C fan/gas 6. Melt the butter and chocolate in a small saucepan over a low heat. Once melted, add the cocoa powder and whisk together until smooth and glossy. Set aside to cool.
In a large mixing bowl, add all other ingredients. Add the chocolate mixture and whisk together until smooth.
Pour the batter into the tin, dollop the peanut butter in the mix and spread to the edges, swirling together with a stick or knife. Bake for 40 minutes, until the top has a light crust but the centre still has a bit of a wobble. Leave to cool completely to firm up before slicing.