Mix the flour, sugar and salt in a bowl. Add the butter and rub together using your hands to form an even, crumbly mixture. Pour in the water, mix quickly to form a soft dough (do not knead), cover and chill for approx. 30 mins. Roll out the dough on a sheet of baking paper (approx. 30 cm in diameter), place on a baking tray.
Mix the plums and vanilla sugar. Spread the ground biscuits on top of the dough, leaving a border of approx. 4 cm all the way around. Arrange the plums on top, fold the edges in slightly, brush with a little water, sprinkle with sugar.
Approx. 1 hr. in the lower half of an oven preheated to 180°C. Remove, allow to cool slightly, slide the tart onto a cooling rack and leave to cool completely.