For the elderberry-apple soup, mix elderberry juice with 250 ml water, 100 g sugar, vanilla sugar and cinnamon in an unperforated cooking container.
Peel, halve and core apples and cut flesh into wedges. Add to elderberry soup and cook (at 100°C for 5 minutes).
In the meantime, beat egg whites until stiff, sifting in powdered sugar and lemon zest.
Season soup with more sugar if necessary. Cut small dumplings from the beaten egg whites and place on top. Allow to set in the residual heat for approx. 5 minutes.