Activate the yeast with 1/2 the cool water, the sugar and a little flour.
Set aside 1 yolk for smearing, beat the other with the yoghurt and remaining water.
Sift the flour, make a well and pour in the 2 mixtures, knead a nice, smooth dough, cover it and leave to rise.
Divide the ready, risen dough into 3 equal parts and 1 smaller one - the latter for forming the center of the pita. Roll out the 3 parts into 3 identical oval sheets, smear the 1st with butter, place the 2nd, smear again and place the 3rd one on top.
Cut into 12 leaves but leave them attached to the center. Twist each leaf left or right.
From the remaining dough, roll out 2 small oval sheets, smear 1 with butter, place the 2nd on top. Cut into 4 parts, make incisions into each and slide the vertex from the bottom going up through it.
Place the small centerpiece on top of the pita, cover and leave to rise, then smear with butter and bake at 320°F (160 °C). These pitas with yoghurt end up very fluffy and threadlike inside, not crumbly like bread.