Grease a fat pan (deep baking tray; approx. 32 x 39 cm). Preheat the oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Chop the chocolate coating and melt with 450 g butter in pieces at low heat while stirring.
Allow to cool for about 5 minutes.
In the meantime, roughly chop the nuts. Sort the blackberries, wash and drain them if necessary. Mix eggs, sugar and 1 pinch of salt with the whisk of the mixer for about 3 minutes until creamy. Stir in the couverture-butter mixture.
Mix flour and baking powder. Stir in quickly. Fold in 200 g nuts.
Smooth the dough on the baking tray. Bake in a hot oven for 10 minutes. Take out, spread berries and the rest of the nuts on top and bake for 20-25 minutes. Let it cool down and cut it into about 30 pieces.