Heat the oven to 220°C, (fan 200°C, gas mark 7). Roll out the dough into an oblong 3 times as long as it is wide. Dice the remaining butter and put one quarter of it over the top two thirds of the dough. Fold the bottom third up and the top third down and turn around so that the folds are now at the sides. Seal the edges by pressing with a rolling pin.