Mix the soft butter with 250 g sugar, vanilla sugar, eggs and red wine. Mix flour, baking powder, cocoa and spices and fold in.
Pour the batter into a greased springform pan and bake in a preheated oven at 170° convection oven for 50 – 55 minutes. Let the cake cool and remove from the pan. Slice the kumquats and remove any seeds.
Bring the remaining sugar to the boil with a good dash of cognac in a small saucepan and reduce the cognac slightly. Add the kumquats, simmer for a few minutes, then drain and let cool. Warm the jam a little so that it becomes spreadable. Cut the cake in half once and spread the bottom layer with the jam.
Set aside a few nice slices of the kumquats and spread the rest on the bottom.
Place the top cake layer on top. Heat the chocolate coating and cover the cake with it. Decorate with chopped pistachios and kumquat slices. It is best to leave the cake for a day.