Let the blueberries thaw slightly. Chop the chocolate. Cut butter into pieces. Roughly chop the nuts. Melt the chocolate and butter in a saucepan over low heat
Beat 2 eggs, brown sugar and salt in a bowl with the whisk of the hand mixer until creamy. Remove the chocolate-butter mixture from the heat and stir in. Mix flour, cocoa, 50 g nuts and baking powder and fold into the chocolate mixture. put about 2/3 of the dough into a greased springform pan (26 cm Ø) and smooth it down
Mix the quark, sugar, 2 eggs and starch with a whisk until smooth. Fold 150 g blueberries into the quark mixture. Carefully add the quark to the dough and spread. Spread the rest of the dough in patches. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 45 minutes
Take out and let it cool down on a cake rack. Puree the remaining blueberries. Cut the cake into pieces and decorate on a plate with puree and chopped hazelnuts
waiting time approx. 2 hours