Preheat the oven to 200°C/400°F/Mark 6. Sprinkle the pastry board and the pastry with flour and roll the pastry out into a disc 25 cm (10 in) across. Lift it on to the tart tin and carefully line the tin. Prick the sides and bottom of the pastry with a fork and trim round the top of the tin with a small knife. Put in the refrigerator to chill.
Stone the prunes and put them to soak for 15 minutes in a bowl of warm water. Meanwhile put the whipping cream, eggs, vanilla sugar, ground almonds and orange-flower water in a large bowl and whisk them together. Melt the butter in a small pan and pour it into the batter, whisking it in thoroughly.
When the oven is ready, after about 15–20 minutes, take the chilled tart shell out of the refrigerator. Drain the prunes and dry them on a cloth or with kitchen paper. Arrange them on the bottom of the pastry and pour the batter over the top. Bake for 25 minutes. When it is done sprinkle the tart with 4tablespoons of Armagnac and serve warm.