Preheat a baking tray in oven at 150C.
Combine Armagnac and prunes in a small saucepan and gently bring to the boil. Very gently simmer for 3 minutes,until liquid has reduced. Remove from heat,cover and stand until cooled to room temperature. Drain prunes in a strainer over a bowl.
Pour cream,sugar and vanilla into a saucepan,gently heat and stir to dissolve the sugar.
In a separate bowl,mix egg and yolks together. Pour the heated cream over the egg mix and stir then strain into a pouring jug.
Halve the prunes lengthways and lightly press in concentric circles into pre-baked tart case.
Place tart onto the hot baking tray then pour the warmed egg and cream mixture gently on top. Bake 15-20 minutes or until the custard is just set. Cool.
Using a hot,clean knife,slice into 10 or 12 portions,making sure to clean the knife between each cut. Serve.
Tip: Serve the tart with whipped cream or ice-cream. However for me,it's best by itself.