For the cake base, cream the butter with the sugar, vanilla sugar and egg. Add flour and baking powder and knead briefly. Form into a ball and chill for approx. 1 hour.
Preheat the oven to 180 °C. Grease a cake tin.
Crush the rusks with a rolling pin and mix the crumbs with the nuts and cinnamon.
Roll out the dough into a thin, round disk and line the tart pan with it. Spread half of the rusk-nut mixture on top and sprinkle the cherries on top. Pre-bake the cake in a hot oven for about 30 minutes.
For the icing, whisk together the eggs, sugar, sour cream, cinnamon and the remaining nut mixture. Cover the cake with this mixture and bake for 30 minutes until done.