Dissolve the yeast and sugar in the slightly warmed milk in a bowl. Add the beaten eggs, yoghurt, oil and salt and stir the mixture. Add the flour in lots, knead a soft and stretchy dough.
Divide the dough into 5 equal parts and shape balls, roll them out into 5 thin equal-sized sheets, 1 at a time. Smear each sheet generously with soft butter, arrange them on top of one another, then wrap them together into a roll.
Put the roll for 30 min. in the fridge. After the allotted time, take roll out and cut into slices 3/4″ (2 cm) thick.
Arrange the slices horizontally in a tray laid out with baking paper, then put the desired filling in their middle - sweet or salty. Fold the slices in 2, like shells, by pressing lightly with your fingers.
Smear the shells with beaten egg yolk + 1 tbsp milk, put the tray in a preheated 392°F (200 °C) oven and bake to a light crisp.