2Heat prunes with enough water to cover in a saucepan to a simmer; simmer until prunes are tender but still hold their shape, about 10 minutes. Drain prunes; arrange in bottom of the prepared baking dish.
3Whisk flour, white sugar, vanilla sugar, baking powder, and salt together in a large bowl. Whisk in eggs, one at a time, beating well after each addition. Gradually pour in milk, stirring vigorously with a wooden spoon until combined. Stir in rum. Pour batter evenly over prunes.
