Roast hazelnut kernels in a pan without fat. Take out and let cool down. Cream butter, 175 g sugar, salt and vanillin sugar with the whisk of the hand mixer. Stir in the eggs bit by bit.
Mix flour and baking powder and stir in alternately with 3 tablespoons of milk.
Wash the grapes, drain them and pluck them from the stems. Chop the hazelnuts coarsely. Fill one half of the dough into a greased box form (11 x 30 cm, approx. 1.75 litres capacity) dusted with flour.
Stir the cocoa, 1 tbsp. milk and 150 g hazelnuts into the second half of the dough, spread on the light-coloured dough. Swirl with a fork, smooth down and spread grapes on top. Bake in a preheated oven (electric cooker: 175 °C/ circulating air: 150 °C/ gas: see
manufacturer) bake for about 45 minutes. Take out and let cool down.
Soak gelatine in cold water. Whip the cream until stiff. Stir mascarpone, 3 tbsp. sugar and white wine with a whisk until smooth. Squeeze the gelatine, dissolve and stir in 4 tbsp. of the mascarpone, stir into the remaining mascarpone.
Gradually fold in the cream. Chill the mascarpone cream for about 30 minutes.
Remove the cake from the mould and place on a cake plate. Spread the mascarpone cream on top and sprinkle with the remaining hazelnuts. Chill for at least 1 hour.