Preheat the oven to 180C. Grease a large (18-20cm) frying pan or oven-proof dish with butter. Set aside.
Beat the butter and both sugars together in a large bowl or a stand mixer fitted with a paddle attachment until smooth and creamy.
Beat in the egg and vanilla, followed by the flour, bicarbonate of soda, and chopped chocolate, reserving a handful for the top, and stir until just combined.
Pour half of the mix into the greased pan and smooth the surface. Pour the peanut butter on top, and smooth out, followed by the remaining half of the cookie mix. Smooth the top and sprinkle the reserved chocolate on the top.
Place the pan in the oven for 25-30 minutes until golden. Remove from the oven and leave to cool before cutting into wedges or triangles.