Beat the yoghurt with the baking soda and leave the resulting mixture to become effervescent. Then, subsequently, add the egg, flour and salt, while beating.
Add the crumbled feta cheese at the end. Lay out the resulting mixture in a tray covered with baking paper. Place pieces of butter on top, press on them lightly so that they sink into the mixture.
Put the tray in a preheated 392°F (200 °C) oven and bake for about 20 min. or until the loaf gains a golden crust.