Pre-heat the oven at gas mark 4. Line two baking trays with grease proof paper. Cream together the butter and sugar until light and creamy. Beat in the vanilla extract and egg. Divide this mixture into two lots.
Add 125g of the flour into one half then add the remaining 95g of flour and the cocoa powder into the second half. Mix until both halves are a soft dough.
Then roll one out at a time between two sheets of grease proof paper to rectangle that is a 1cm in thickness. Once both are rolled out, remove the top layer of grease proof paper and put one on top of the other.
Then roll up and wrap in cling film. Leave in the fridge for an hour then take out and cut the roll up into about 5mm slices. Put these on the baking trays and bake for 15-20 minutes or until golden. Once done, leave to cool on a wire rack.