Line the bottom of a springform pan (26 cm Ø) with baking paper. Separate the eggs. Beat the egg whites, 3 tbsp. cold water and 1 pinch of salt until stiff, adding 100 g sugar. Fold in the egg yolks one by one. Sift flour and baking powder on top and fold in. Brush into the mould. Bake in the preheated oven (electric cooker: 175 °C/circulating air: 150 °C/gas: level 2) for about 20 minutes. Cooling down
Clean and wash the rhubarb, weigh 600 g rhubarb and cut it diagonally into pieces (approx. 1 cm). Bring 100 ml juice and 2 tablespoons sugar to the boil. Add the rhubarb and cook for about 3 minutes. Lift out about 20 pieces for decoration. Boil the rest of the rhubarb for about 2 minutes. Mix starch and 5-8 tbsp. juice. Stir so much starch into the rhubarb until it is bound. Boil through briefly. Stir in 3 tbsp. sugar. Cool down for about 20 minutes.
Cut the cake base in half. Close a cake ring around the lower cake base. Spread the compote on top. Chill for about 30 minutes.
Soak the gelatine in cold water. Mix yoghurt and 75 g sugar. Squeeze the gelatine and dissolve at low heat. Stir in 3 tbsp. yoghurt, then stir into the rest of the yoghurt. Chill for about 5 minutes until it starts to gel. Whip 100 g cream until stiff and fold in. Spread on the compote. Place 2nd base on top. Chill for about 4 hours.
Whip 200 g cream until stiff, allowing vanilla sugar to trickle in. Spread the cake with 2/3 cream. Decorate with the remaining cream, rhubarb and pistachios