Roughly chop the chocolate and melt over a warm water bath. Beat the fat, 225 g sugar, 1 pinch of salt and vanillin sugar in a mixing bowl with the whisks of the hand mixer until creamy.
Stir in the eggs one by one. Stir in liquid chocolate. Mix flour, cocoa and baking powder and stir in alternately with red wine and sour cream. Colour with living-agent colour. Pour the dough into a greased springform pan (26 cm Ø), dust with flour.
Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 45-55 minutes. Remove from oven and let cool for about 2 hours.
In the meantime soak the gelatine in cold water. Whip the cream with the whisk of the hand mixer until stiff. Mix quark, sour cream, cream and 125 g sugar. Squeeze the gelatine, dissolve carefully.
Stir in 2-3 tablespoons of cream, then stir everything into the rest of the cream. Cut the cake twice horizontally.
Clean and wash the grapes and pluck 450 g from the vine. Place a cake ring around a base. Spread 1/4 of the cream on the base, spread half of the grapes on top. Place the middle cake base on top.
Add another 1/4 of the mixture and spread the remaining grapes on top. Finish with the cake lid. Chill the cake for about 1 hour.
Caramelise 20 g sugar in a pan and add walnuts. Pour the caramelised nuts onto a baking tray and allow to cool. Remove the cake from the cake ring and spread with the remaining cream. Decorate the upper cake ring with nuts.
Dust the middle of the cake with cocoa. Decorate with remaining grapes.