Prepare the börek filling.
To make the dough, melt the butter and pour into a bowl with the egg, water, vinegar and salt. Mix well, add both flours and knead to a smooth dough. Divide into two lumps, place them on a floured tray and allow to rest for 20 minutes under a damp cloth. On a floured pastry board roll out one of the lumps of dough to a thick strip with a long, thin rolling pin