For the plum cake, stir butter, powdered sugar and vanilla sugar until smooth. Then add the yolks and continue to beat until smooth. Stir in a pinch each of salt and cinnamon and stir in QimiQ.
Mix the flour and baking powder together and fold into the mixture 3/4 at a time. Finally, alternately fold in the stiff beaten egg whites with the remaining 1/4.
Spread the mixture on a baking sheet and place the quartered plums on top. Bake the plum cake at 160° for about 30 minutes.