Mix flour and baking powder and sieve into a bowl. Add vanillin sugar, salt, sour cream and softened butter. Knead with the dough hook of the hand mixer, then quickly work into a smooth dough with your hands. Wrap the dough in foil and refrigerate briefly. Meanwhile, for the crumbles, mix flour, sugar, vanillin sugar and cinnamon in a bowl. Add cold butter in small cubes and knead with your hands to a crumbly dough (crumbles).
Divide the kneaded dough into three and roll out on a greased springform pan (28 cm Ø). Make sure that the pastry base is not too thin at the edges and prick it several times with a fork. Sprinkle with 1/3 of the crumbles and bake in a preheated oven (electric: 200 °C/ gas: level 3) for 15-20 minutes. Remove the finished cake base immediately from the springform pan and let it cool on a cake rack. Bake two more cakes one after the other from the remaining dough and the crumbles. Cut one of the cakes while still hot into 12 cake pieces. Spread the two whole cakes with plum puree. Whip the cream, sugar, vanilla sugar and cream setting agent until stiff. Pour the cream into a piping bag with a large star-shaped spout. Spray 12 thick cream tuffs onto the edges of each of the two whole cake layers. Spray the rest of the cream into the middle of the cake layers. Place the two cake layers on top of each other and place the cut cake pieces diagonally on a cream tuff.
Whip the cream, sugar, vanilla sugar and cream setting agent until stiff. Pour the cream into a piping bag with a large star-shaped spout. Spray 12 thick cream tuffs onto the edges of each of the two whole cake layers. Spray the rest of the cream into the middle of the cake layers. Place the two cake layers on top of each other and place the cut cake pieces diagonally on a cream tuff. Refrigerate for about 1 hour. Just before serving, dust with icing sugar and cut into cake pieces with a saw knife. Makes 12 pieces