Put all ingredients for the pastry in a bowl, mix & knead w/ your hands until all is combined. Form a ball, wrap in clingfilm & put it in the fridge (fresh department at about 0° C) for about 30 min.
In the meantime clean, trim & pit the plums; cut them lengthwise into quarters. Add all the other ingredients for the filling & mix thoroughly.
Preheat the oven to 180° C fan. Place a baking tray in the oven.
After the session in the fridge flatten the pastry w/ a rolling pin between 2 layers of clingfilm until almost round w/ a diameter of about 30 cm.
Remove the clingfilm on top & add a sheet of baking parchment. Turn over & remove the 2nd clingfilm.
Put the plum mess in the centre of the pastry & leave a rim of 4 - 5 cm. Fold in the pastry rim.
Now whip the egg and brush the egg wash all over the folded rim.
Open the oven, pull out the baking tray & let the baking paper w/ the galette slip onto the hot baking tray. Let it bake for about 30 min.
When ready and still hot sprinkle the vanilla sugar all over the plums – and serve as soon as possible.