For the dough, warm milk lukewarm. Crumble the yeast and dissolve in it with 2 tablespoons of sugar. Put 500 g flour, salt, quark, butter and eggs in a bowl. Add yeast milk and knead to a smooth dough with the dough hooks of the mixer.
Cover and leave to rise in a warm place for about 45 minutes.
Form about 20 balls from the dough with floured hands. Leave to rise for about 15 minutes on a floured work surface. Heat the oil in a deep fryer or a wide pot to about 160 °C.
Bake the balls for 4-5 minutes in portions, turning them from time to time. Take them out with a skimmer and immediately turn them in 75 g sugar. Leave to cool.
For the filling, stir the jam until smooth, possibly passing it through a sieve. Fill yoghurt and jam into 1 freezer bag each, cut off a small corner. Drill an approx. 1 cm deep hole in each of the quark balls with a wooden stick.
Fill each ball with some jam and yoghurt.