Mix egg white, icing sugar, oil and 50 g flour. Pour into a small piping bag and spray snowflakes of different sizes onto a baking tray (approx. 32 x 39 cm) lined with baking paper.
Place the baking tray in the freezer for about 15 minutes.
Separate eggs. Beat the egg whites with the whisks of the hand mixer until stiff. Add salt, 75 g sugar and vanillin sugar. Stir in egg yolks. Mix 50 g flour, starch, baking powder and cocoa, sieve onto the egg mixture and fold in.
Remove the baking tray from the freezer. Spread the sponge cake mixture evenly on top. Bake in the preheated oven (electric cooker: 220 °C/ convection oven: 190 °C/ gas: see manufacturer) for 8-10 minutes. Take out the sponge cake and turn it over onto a tea towel sprinkled with sugar.
Carefully remove the baking paper. Roll up the sponge cake from the long side using the tea towel and let it cool down.
Soak gelatine in cold water. Finely chop the chocolate. Mix lemon juice, lemon zest, 100 g sugar, sour cream and chocolate. Squeeze the gelatine, dissolve and stir in 4 tbsp. of sour cream.
Stir the gelatine into the remaining silt. Chill until the mass begins to gel. Whip the cream until stiff, fold into the mixture in portions.
Unroll the sponge cake, spread the cream evenly on top. Carefully roll up the sponge cake, place it on a plate with the seam facing down. Chill for about 3 hours. Whipped cream tastes good with it.