Turn on the oven to preheat to 140 degrees. Cover two large baking sheets with parchment. Draw two circles with a diameter of 23 cm on each.
In a large bowl, beat the egg whites until fluffy, firm foam. Add sugar gradually, beat on high speed until stable peaks. Add ground nuts and cinnamon to the proteins.
Place the dough on the drawn circles and place in the preheated oven. Dry the cakes for about 1 hour, until crisp.
Remove the meringue cakes from the oven, allow to cool at room temperature for 10 minutes, then remove from the parchment and allow to cool completely.
Cut the plums into wedges.
Prepare the cream for the meringue cake. Beat the cream with powdered sugar until stable. Add sour cream and vanilla, beat well again.
Collect the meringue cake. Smear the meringue cakes with butter cream and place the plum wedges.