Mix the flour, sugar, vanilla sugar, and eggs in a large mixing bowl. Gradually pour in the milk while whisking until the batter is smooth and lump-free. Then add 10 cl of rum.
Let the mixture rest for 1 hour at room temperature.
Preheat the oven to 200°C. Melt a little butter in a baking dish—preferably a terracotta one—then spread the butter over the bottom and sides. Pour the batter into the dish.
Roll the prunes in a little flour to keep them from sinking to the bottom of the pan, then distribute them throughout the batter.
Bake for 30 minutes at 200°C. Then turn off the oven and let it bake for another 30 minutes in the turned-off oven.
