Beat 4 egg yolks with 4 tablespoons water, 125 g sugar, and 1 packet vanilla sugar until frothy. Mix 125 g flour with 1 teaspoon baking powder and gently fold into the egg yolk mixture.
In a separate bowl, beat 4 egg whites until stiff peaks form. Carefully fold into the batter, then spread onto a parchment paper-lined baking sheet.
Bake at 180°C (356°F) with top and bottom heat for 15 minutes on the middle rack.
Turn the sponge cake over onto a damp cloth, peel off the baking paper, roll up with the cloth, and let cool completely.
For the filling, whip the cream with 1 packet cream stabilizer and 1 packet vanilla sugar until stiff. In a separate bowl, mix the quark with sugar, 1 packet cream stiffener, and lemon juice.
Stir the whipped cream mixture and quark mixture together.
Unroll the cooled sponge cake, fill with the cheese cream filling, and roll up again.