Dissolve 1/2 cube of yeast in 2/5 cup (100 ml) warm water with 1 tsp sugar and a few tablespoons of flour. Wait about 10 min. for the yeast to activate.
Separately, in a deep container, crumble 7 oz (200 g) feta cheese, add 1 beaten egg, the yoghurt and 1 tsp salt.
Add the mixture to the effervescent yeast and stir well so they mix thoroughly. Add flour gradually until you obtain a soft, nonstick dough.
Leave it to rise (double in volume). Divide up the ready dough into 12 balls, from which you'll be forming the buns. You can make them either ball or pleat-shaped.
Put 1 tsp crumbled feta, mixed with egg white, in the middle of each bun. Leave them to rise well for about 30 min.
Put a piece of butter on top of each. Smear with egg yolk mixed with oil. Bake at 356°F (180 °C).
Once they redden well and are well baked, sprinkle them with grated cheese. Turn the oven off and leave them inside for the cheese to melt.