Break chocolate into pieces. Melt with butter at low heat, stirring occasionally. Finely chop the berry mixture. Mix flour, cocoa and baking powder. Whisk the egg. Pour chocolate butter into a mixing bowl.
Add sugar, egg, berry and flour mixture. Stir with the whisks of the mixer to a mass. Chill for about 15 minutes.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Line two baking trays with baking paper. Sieve icing sugar onto a deep plate. Form cherry-sized balls from the dough.
Turn in portions in the icing sugar. Spread it out on the trays at a distance. Bake in a hot oven for 12-15 minutes one after the other. Let them cool down on cake racks.