The crucial aspect of this dough is that it's kneaded using feta cheese brine. It needs to be high quality - made from strained whey and salt, it needs to look clear, with a pleasant aroma.
Put the flour in the container you intend to knead the dough, make a well. Pour in the yeast and brine that you've dissolved in a little sugar and cool water. Knead a soft dough, while adding cool water until all of the flour is absorbed. The dough must not stick.
Wrap it with a towel and leave in a warm area about 15 min. Roll it out onto a floured surface to 1/2″ (1 cm) thick. Using a sharp knife, cut the sheet down the middle into 2 equal halves.
Smear the surface with the egg white, set the yolk aside for smearing later. Crumble the feta cheese on top evenly. Wrap them up into rolls.
The 2 halves yield 2 rolls, now cut each into 3/4″ (2 cm) thick pieces. Arrange the chopped pieces lying on their sides in an oiled tray - they look like snail shells.
Leave them covered with a towel in a warm area to rise. Smear them with beaten egg yolk and put a little butter on each piece.
Bake in a moderate oven until reddened. Take them out, sprinkle with a little cold water and cover with a towel for about 10 min.
Serve warm with homemade yoghurt or ayran.