Wash the grapes, pluck them from the panicles and let them drain well. For the icing, mix sour cream, 4 eggs, 75 g sugar and sauce powder with the whisks of the hand mixer. Whip 250 g sugar, salt, vanillin sugar and fat with the whisk of the hand mixer until creamy white.
Add 4 eggs one after the other and stir in. Mix flour and baking powder and stir in. Put the dough on a greased fat pan of the oven (32 x 39 cm), smooth it down and spread the grapes on top.
Pour the cast iron evenly on top. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 40 minutes. Remove from the oven, place on a cake rack and allow to cool.
Dust with icing sugar.