Grease a square springform pan (24 x 24 cm). Mix flour, cocoa and baking powder. Preheat the oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer).
For the dough, melt 250 g butter in a saucepan over a low heat. Break the chocolate into pieces, add and melt in the warm butter while stirring.
Beat the eggs, sugar and 1 pinch of salt in a large bowl with the whisk of the mixer for approx. 5 minutes until thick and creamy. Stir in the warm chocolate-butter mixture immediately. Stir in the flour mixture only briefly.
Fill the dough into the mould and smooth it down. Bake in a hot oven for about 25 minutes. The brownie should not be completely baked through, so there are still traces of dough on the wooden skewer when you make a stick test.
Remove and let it cool down on a cake rack.
Select the blueberries, wash and drain well. Stir the jam until smooth and add to the blueberries. Mix cream cheese and sugar. Whip the cream until stiff, adding cream firming agent and vanilla sugar.
Fold the cream into the cream cheese.
Remove the brownie from the mould, place the edge of the mould around the bottom again. Spread the cream on the bottom. Add blueberries, spread and chill for approx. 2 hours. Remove from the mould and cut into pieces.