Sift the flour with the salt. Make a well in the middle and add the egg and the yoghurt, in which you've extinguished the baking soda. Knead a soft dough, leave it for about 30 min. to "rest".
Tear a ball the size of a mandarin from the dough and roll it out into a thin sheet. Put it in an oiled tray, sprinkle with feta cheese, cheese and butter and bake in a preheated 356°F (180 °C) oven until the cheese crisps (about 10 min.).
Repeat this process until out of ingredients. Serve the flatbread warm.