Mash the potatoes or put them through a press. Mix with sugar, quark, flour, baking powder, egg yolk, cranberries, lemon zest and a pinch of salt. Beat the egg white until stiff and fold in carefully. Put the dough in the refrigerator for 20 minutes.
During this time you can prepare the sauce. Strain the blueberries, collecting the juice.
Bring the juice with the cinnamon stick and powdered sugar to the boil and simmer for 5 minutes.
The cornstarch in 1 tablespoon. Dissolve the water and stir into the blueberry juice. Keep simmering until the juice starts to thicken. Then the blueberries come to the juice. Reheat the sauce and then keep it warm, as it is served warm with the quark pods.
Melt the butter in a pan on a low flame and slide 1 tablespoon of dough into the pan for a quark pudding. Turn the quark cakes after 2 minutes and bake for another 2 minutes.
Serve the cottage cheese with the warm blueberry sauce.