Preheat oven to 180 °C top/bottom heat. Line a baking tray with baking paper.
For the sponge coat, beat egg whites with a pinch of salt until firm.
Beat yolks with powdered sugar, lemon zest and vanilla sugar until very foamy. Quickly stir half of the egg whites into the yolk mixture.
Mix flour with cocoa powder, sift onto a piece of baking paper and stir into the egg mixture. Carefully fold in the remaining snow and spread the sponge mixture evenly (approx. 25 x 25 cm) on the baking paper. Bake in the preheated oven for about 8-10 minutes.
Remove from the oven and sprinkle thinly with powdered sugar, place a clean kitchen towel on top and turn the sponge over with the baking paper. Carefully remove the baking paper from the dough.
Cut the sponge cake into 5 strips. Cut 4 circles out of one of the strips with a round cookie cutter (about 5 cm in diameter).
For the mousse, soak the gelatine in cold water for about 10 minutes.
Mix yogurt with powdered sugar. Whip the cream until stiff.
Squeeze gelatine, melt according to package instructions and stir into yogurt. Fold in whipped cream and chill for approx. 10-15 minutes.
In the meantime, cut 4 strips of baking paper about 15 cm wide, fold them lengthwise and put them together.
Place a sponge cake strip on each piece and roll up to form a ring of sponge cake. Secure with a paper clip or wooden clothespin and place on a flat plate. Use the remaining circles as a base in d