Starter: In a small bowl, mix 2 tbsp flour, 1 tbsp sugar, 50 ml warm milk, and 40 g fresh yeast. Cover with a cloth and let rise for 20 minutes.
Dough: Combine the remaining flour, sugar, milk, yolks with yogurt, salt, and melted butter. Add the starter and knead for about 7 minutes until smooth. Let rise for 1 hour in a warm place.
Filling: In a pan, cook diced apples with butter, sugar, cinnamon, nutmeg, and vanilla sugar for about 10 minutes, stirring often. (Optionally add jam or apple purée.)
Shaping: Divide the dough into 12 balls. Roll each into an oval. Place apple filling (and jam if using) on one half, fold over like a turnover, cut strips into the other half, and fold them over. Tuck ends under to form crescents.
Baking: Place crescents on trays (6 per tray), let rise for 15 minutes. Brush with yolk and milk mixture. Bake for 10 minutes at 180 °C (356 °F).
Finishing: Once cooled, dust with powdered sugar mixed with cinnamon.