Step 1

Preheat the oven to 450 degrees F (230 degrees C). Grease a shallow 2 quart baking dish.

Step 2

Place the pitted prunes in a saucepan with enough water to cover. Bring to a simmer, and cook until the prunes are tender but still hold their shape, about 10 minutes. Drain the prunes and arrange them in the bottom of the prepared baking dish.

Step 3

Whisk together the flour, white sugar, vanilla sugar, baking powder, and salt in a large bowl. Whisk in the eggs one at a time, beating well after each addition. Gradually pour in the milk, stirring vigorously with a wooden spoon until combined. Stir in the rum. Pour the batter evenly over the prunes.

Step 4

Bake in the preheated oven for 10 minutes. Reduce the oven temperature to 400 degrees F, and bake until the filling is firm, has risen a little, and the top is browned, about 30 minutes. Serve warm or at room temperature.