In a saucepan, warm the Armagnac over medium heat. Remove from the heat, add the prunes, and let steep, stirring occasionally, until softened, 1 to 2 hours. Drain and reserve the steeping liquid.
Preheat the oven to 375 degrees F.
In a bowl, beat the egg with 1/3 cup of the sugar until thick, about 2 minutes. Add the flour and 1 cup cream, and beat well. Add 3 tablespoons of the reserved prune soaking liquid and beat well.
Arrange the prunes in the bottom of the prepared tart shell. Pour the cream mixture over the top and bake until risen and golden brown, 25 to 30 minutes. Remove from the oven and cool on a wire rack.
In a bowl, whip 3/4 cup cream with an electric mixer until soft peaks just start to form. Add the remaining tablespoon of sugar and 1 to 2 tablespoons of the remaining prune liquid, and whip until soft peaks form. Transfer to a decorative bowl.
To serve, sprinkle with powdered sugar and serve with the Armagnac cream on the side.
Yield: 1 (9-inch) tart, 6 to 8 servings