Bring the cream to the boil, remove from the stove. Break chocolate into pieces and melt 400 g in it while stirring. Let it cool down. Then cover and chill for about 4 hours.
Grease a fat pan (approx. 32 x 39 cm; at least 3.5 cm deep) and dust with flour. Chop the nuts. Dice butter. Melt 400 g chocolate pieces and butter in a hot water bath, cool down a little. Beat eggs, 1 pinch of salt and sugar for 8-10 minutes until fluffy. Gradually stir in the butter and chocolate mixture. Mix flour and baking powder and stir in portions briefly. Fold in nuts
Spread the dough on the fat pan. Bake in the preheated oven (electric cooker: 150 °C/circulating air: 125 °C/gas: level 1-2) for approx. 35 minutes. Let them cool down
Cut the brownie base in half lengthwise to make 2 smaller plates. Whip the chocolate cream until stiff, spread on the plates. Chill for about 1 hour. Draw diagonal lines in the cream with the back of a knife. Shortly before serving, cut into pieces and dust with cocoa