Separate eggs. Beat egg white and salt until stiff. Add 100 g sugar and continue beating. Stir in the egg yolks briefly. Mix flour, starch and baking powder and sieve onto the egg mixture. Add 75 g poppy seeds and fold in carefully.
Line the bottom of a springform pan (24 cm Ø) with baking paper. Put the dough into the tin and smooth it down. Bake in the preheated oven (electric: 175 °C / gas: level 2) for about 40 minutes. Let the base cool down on a cake rack.
Then remove from the springform pan and cut through 2x. Soak gelatine in cold water. Sort blueberries, wash carefully and drain well. Whip 1 1/2 cup of cream and vanillin sugar until stiff.
Squeeze the gelatine, dissolve at low heat and stir into the cream. Fold in 250 g blueberries. Place the bottom cake layer on a plate and place the edge of the springform pan on top. Spread half of the blueberry cream on the cake base, mix with the 2.
Cover the ground. Spread the rest of the blueberry cream and place the 3rd cake layer on top. Chill the cake for about 1 hour. Remove the edge of the springform pan. Whip remaining cream and sugar until stiff. Spread 2/3 of the cream all around the cake.
Fill the rest of the cream in a piping bag with a star-shaped spout and squirt onto the cake in circles of about 2 cm apart. Decorate the cake edge with a serrated comb and sprinkle with the remaining poppy seeds. Sprinkle remaining blueberries in the spaces between the circles and serve the cake.
Results in about 16 pieces.