Break 225 g chocolate into pieces. Dice the butter. Melt both in a water bath over low heat.
Roughly chop 75 g chocolate. Beat eggs, sugar and flavor for about 8 minutes until frothy. Sieve the flour, cocoa and baking powder on top, fold in carefully. Then chocolate butter and sour cream, finally fold in the chopped chocolate.
Spread the mixture on a deep baking sheet lined with baking paper and bake for about 20 minutes at 175 degrees. (Circulating air: approx. 150 degrees).
Let the cake cool, cut into pieces and serve with whipped cream.