Beat the softened butter with the sugar and a pinch of salt using a mixer.
Add the egg, then the sour cream, and keep beating.
Add the flour and baking powder and knead the dough. Add more flour if it is too sticky, but it should not become stiff. Roll into a ball and chill while you make the filling.
For the filling, mash the curd cheese with the sugar and vanilla sugar using a fork. Add the yolk and raisins and mix. Raisins can be replaced with dried cranberries, chocolate chips or dried cherries.
If the curd cheese is very wet, add another tablespoon of flour to the filling.
Dust the work surface with flour, pinch off small balls of dough and roll each into an oval. Put the filling on one half and fold the other half over it.
Brush all the turnovers with egg yolk and bake for 25–30 minutes in an oven preheated to 180°C.