Grease a springform pan (approx. 20 cm Ø) and sprinkle with breadcrumbs. Finely grate the chocolate. Mix with nuts and baking powder. Separate the eggs. Beat egg whites until stiff.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Cream 80 g butter, sugar, vanilla sugar and 1 pinch of salt with the whisks of the mixer. Stir in egg yolk. Stir in the chocolate-nut mixture (the dough becomes very firm).
Stir in half the beaten egg white first. Then fold in the remaining beaten egg white in portions.
Fill the dough into the springform pan and smooth it down. Bake in a hot oven for approx. 45 minutes (test with wooden sticks). Remove from the oven and place on a cake rack. Carefully remove the cake from the edge of the springform pan with a knife and let it cool down in the pan.
Remove from the mould. Dust with icing sugar.