For the prunes, cut prunes into small cubes, mix with plum brandy in a saucepan, cover and let stand overnight. Simmer prune mixture over low heat until the liquid cooks away, remove from heat and let cool.
For the batter, beat butter until creamy and gradually stir in powdered sugar, vanilla sugar and eggs. Mix flour with baking powder and stir into batter mixture. Stir prune mixture into batter.
Pour batter into buttered ramekins about 5 cm diameter (approximately 2 inches diameter) and bake tarts in oven preheated to 180°C (fan: 160°C, gas mark 2-3)(approximately 350°F) for 15-20 minutes. Serve prune tarts immediately or let cool.