Preheat the oven to 175°C top/bottom heat.
Beat the eggs with the sugar. Add the remaining ingredients for the sponge cake and fold in by hand.
Pour the batter into a ø20cm baking pan and bake for 30 minutes until golden brown.
Meanwhile, pit the plums and boil them together with the preserving sugar and cinnamon. Optional: puree.
Remove the cinnamon sticks, cut the cake once and fill it with the plum puree.
Finally, mix the guma with the cinnamon and sugar for 3 minutes and brush the top of the tart with it.
Sprinkle with cinnamon and enjoy.