Put blueberries in a bowl, sprinkle with 125 g sugar and defrost. Grease the fat pan of the oven (deep baking tray; approx. 32 x 39 cm) and dust with flour. Preheat the oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see
Separate eggs. Beat egg whites and 1 pinch of salt until stiff and chill. Whisk 350 g butter and 350 g sugar with the whisks of the mixer until white and creamy. Stir in the egg yolks one by one. Mix 350 g flour and baking powder, stir in 150 g yoghurt briefly.
Carefully fold in 3 portions of egg white. Put the dough into the fat pan and smooth it down. Bake in a hot oven for about 20 minutes.
For the organic orange syrup, wash hot, dry and rub the peel. Squeeze all oranges and measure 300 ml juice. Bring the orange juice, zest and 150 g sugar to the boil and boil down over a high heat for 6-8 minutes until syrupy.
Take out the cake and sprinkle with the still hot syrup. Let it cool down on a cake rack.
For the cream, soak gelatine in cold water for about 5 minutes. Crush the blueberries slightly with a fork and stir in 500 g yoghurt. Squeeze the gelatine and dissolve at low heat.
First stir 2-3 tablespoons of yoghurt cream into the gelatine, then stir everything into the remaining cream. Put it in a cold place. Whip the cream until stiff and fold in as soon as the cream begins to gel.
Halve the bottom crosswise. Spread blueberry cream evenly on the cake base and smooth it down. Chill for about 2 hours. Chop the pistachios. Cut the cake into pieces and sprinkle with pistachios.