Preheat the oven to 160° C fan.
Butter the tart form.
Rinse the plums, trim the plums, halve each plum & discard the pit. Then cut the plums into small slices. Set aside.
Mix sugar, vanilla sugar & the ground cinnamon (optional). Add a pinch of salt.
With the electric handheld mixer: mix the eggs until fluffy. Add the sugar mix & mix until fluffy.
Sift the flour onto the egg-sugar mess and mix until well combined. Avoid any lumps of flour!
Pour the flour mess into the tart form.
Distribute the plums evenly all over the flour mess.
Bake for at least 45 min.
Sprinkle some icing sugar generously all over the still warm clafoutis.
Serve warm - or at least at room temperature.