Take the phyllo pastry, yoghurt and eggs out 5-10 min. ahead of time, so that they're at room temperature.
Beat the eggs in a bowl. Shake the yoghurt cup and put the baking soda in it. Stir with a spoon until it bubbles. Add the yoghurt to the eggs, add the carbonated water and stir until homogenized.
Crumble the feta cheese in a separate plate.
Grease an oven dish and put 2 phyllo pastry sheets. Put in a ladle of the mixture and some feta cheese.
Repeat this procedure until you run out of mixture and phyllo pastry sheets.
Pour on a beaten egg yolk on top and then bake for 30 min.