Place the flour, dough, oil, yeast, milk, salt and water in the bowl of an electric mixer and, with the dough hook, mix for around 6–8 minutes, until smooth and elastic. Cover the dough and leave to prove for just 10 minutes. Then divide it into 12 balls and flatten out each one. Place a square of chocolate in the middle of the dough and pull the dough around it so it is completely enclosed. Reshape the dough into a ball and place on a floured baking tray. The buns will spread, so be sure to allow a little room between each one. Dust with cocoa powder, cover loosely with cling film and prove in a warm place for 2 hours.