It is best to prepare this dessert a day in advance.
In a mixing bowl, beat the egg yolks, sugar and vanilla sugar until frothy. In another mixing bowl, beat the cream mixed with the cinnamon until stiff. Now fold the cream into the egg yolk and sugar mixture. Fill the parfait mass into a flatter rectangular shape and place it in the freezer overnight.
Choose 2 nice-looking plums per person and puree the remaining plums (without the juice) to make a mousse.
Take the cinnamon parfait out of the freezer about 3 minutes before serving (just let it thaw slightly) and cut into slices. Put a portion of plum sauce on each dessert plate and use a wooden stick to draw a nice pattern into it. Then add one or two slices of parfait and place the 2 plums that you put back on top.