Melt butter and chocolate coating in a pot. Put the butter-chocolate mixture into a mixing bowl. Add 3 eggs and mix with the whisk of the hand mixer. Mix 140 g sugar, flour, baking cocoa, baking soda and baking powder and stir into the dough by the spoonful until a smooth mixture is obtained. Fold the hazelnuts into the dough. Grease a box form (11 x 25 cm), fill the dough into the form and smooth it down.
Mix quark, sour cream, 3 eggs, 250 g sugar and starch. . Fold blueberries into the cheese mixture. Pour onto the brownie dough and bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 1 hour and 15 minutes.
During the whole baking time, place a small metal bowl with some water on the bottom of the oven. Switch off the oven. Open the oven door slightly and let the cake cool down in the switched off oven for about 20 minutes. Then take the cake out of the oven and let it cool down completely.