Caramelize 2 tablespoons of powdered sugar, mix in the chopped walnuts and let cool on an oiled aluminum foil. Peel, core and dice the apples. Briefly sauté with the lemon juice and elderflower syrup. Mix the quark, yoghurt and 2 tablespoons of powdered sugar. Crumble the cantuccini. Layer alternately apples, caramel, quark yoghurt mixture and cantuccini in four glasses. Finish with caramel.