Mix the quark with the egg yolk and the vanilla sugar (you can alternatively use 2 tablespoons of homemade vanilla sugar) until creamy. Stir in most of the lemon zest and lemon juice. Finally, mix the lemon balm leaves into the curd mixture with a hand blender, so that a homogeneous light green cream is created. Now fill into ice cream cups or beautiful glasses and let them cool in the refrigerator.
Just before serving, garnish with the rest of the lemon peel and a lemon balm leaf.