Whisk the butter, powdered sugar and vanilla sugar until fluffy. Add the egg yolks, salt and cinnamon and mix well.
Whisk QimiQ Classic smooth, add to the cake mixture and mix well.
Sift the flour and baking powder together and fold three quarters into the above mixture.
Whisk the egg whites and sugar stiff and fold into the mixture with the remaining flour.
Spread onto a baking tray lined with baking paper and arrange the halved plums on top of the cake. Bake in the middle of the oven for 20-30 minutes. Dust with the powdered sugar and cinnamon mixture whilst warm.