Put the water with the yeast, sugar and 2 tbsp flour in a bowl, mix and wait for the yeast to activate.
Sift the rest of the flour, add the whipped eggs, oil, salt, effervescent yeast and yoghurt (in which you've put the baking soda beforehand). Stir the mixture well. It will become thick and sticky. Leave it to rise.
Butter a tray and pour in half of the mixture, sprinkle with feta cheese and melted butter and pour in the rest of the mixture on top. Even it out with a buttered spoon.
Throw on the rest of the butter, in pieces, on top. Bake at 338°F (170 °C), without preheating the oven ahead of time.