Arrange the plums in an tart pan with the skin side down (you can also use another type of pan if you prefer). Sprinkle approximately 1 Tbsp of vanilla sugar on top of the plums, and place the pan in the middle of the oven. Safe the the rest of the vanilla sugar for later use, or use it to mix into some greek yoghurt and eat in on the side with the baked plums.
Bake the plums for approximately 1 hour and 30 min, until the plums have curled up a little bit and look slightly baked and tender.