Preheat the oven to 450ºF. Grease a large, ovenproof dish with the salted butter.

Combine the flour, sugar, vanilla sugar, baking powder and the pinch of salt in a large mixing bowl. Give the mix the shape of a well and reserve.

Put the prunes in a saucepan with the milk and the rum. Bring to a boil on medium heat. The prunes should be tender but should still hold their shape. Drain the prunes and place them into the dish.

Add the egg to the flour mixture, starting from the center, and whisk to incorporate the flour. Gradually add the boiled milk to the mix, whisking vigorously until combined.

Carefully pour the batter over the prunes, then place in the preheated oven and bake for about 10 min at 450ºF and for about 40 minutes at 350ºF. It is cooked when the top is browned, and the filling has risen and is firm. Serve warm or at room temperature.