Line the bottom of a springform pan (26 cm Ø) with baking paper. Separate the eggs. Beat the egg whites until stiff, add 125 g sugar, 1 packet of vanillin sugar and salt. Stir in egg yolks and 50 g cream briefly. Mix flour, cocoa and baking powder. Sift over the egg cream and fold in carefully. Pour the mixture into the springform pan and smooth it down. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for about 25 minutes. Let it cool down on a cake rack.
For the compote, wash, clean and cut rhubarb into pieces. Mix the starch with 50 ml juice until smooth. Bring 150 ml juice, rhubarb and 2 tablespoons sugar to the boil in a pot. Cover and simmer for about 5 minutes. Stir in the starch and let it simmer for about 1 minute while stirring. Remove from the heat, fill into a bowl and let it cool down.
Remove the base from the pan and remove the baking paper. Cut the cake twice horizontally. Place a cake ring around the lower cake base. Mix 2 sachets of vanilla sugar and cream thickener. Whip 500 g cream until stiff, allowing the cream firming mixture to trickle in. Fold in the compote briefly. Spread half of the cream on the bottom cake layer and smooth it down. Place the second cake layer on top. Add the rest of the cream and spread. Place the top layer on top and chill for about 1 hour.
Chop the chocolate and melt over a warm water bath. Mix 2 tablespoons of cocoa and 2 tablespoons of sugar. Whip 750 g cream until stiff, pour in the cocoa mixture. Stir 3-4 tbsp. of the cream into the melted chocolate. Quickly fold the remaining cream into the chocolate mixture in 2-3 portions. Spread the cake with about 1/3 of the cream. Pour the remaining chocolate cream into a piping bag with a large perforated spout.
Spray the edge of the cake vertically with 3 spots. Press in with a spoon, squirt the spots again. Repeat the process. Spray spots on the top of the cake as well. Dust with cocoa and decorate with chocolate rolls. Chill for about 1 hour.