Preheat the oven to 450°F (230°C). Grease a shallow 2-quart baking dish.
STEP 2
Heat prunes in water to simmer; drain and arrange prunes in baking dish.
STEP 3
In a bowl, whisk flour, sugar, vanilla sugar, baking powder, and salt. Whisk in eggs one at a time. Gradually stir in milk and rum. Pour batter over prunes.
STEP 4
Bake for 10 minutes. Reduce temperature to 400°F (205°C); bake until firm and browned, about 30 minutes. Serve warm or at room temperature.