Bring the milk and vanilla bean to the boil in a heavy-based saucepan over a low heat. Whisk together the egg yolks and 35g sugar in a bowl until pale and doubled in volume. Whisk in the flour and continue to whisk until the mixture is smooth. Pour a third of the boiled milk onto the egg mixture and whisk thoroughly. Pour this into the saucepan with the remaining milk, return to a low heat and cook for 5 - 6 minutes stirring continuously until the pastry cream has thickened. Remove from the heat and pass through a fine chinois, discarding the vanilla bean. Sprinkle icing sugar onto the surface of the pastry cream to prevent a skin from forming. Refrigerate until ready to use.
Melt the chocolate over a bain marie. Cream together the butter and remaining sugar and stir in the egg, almond meal and cocoa powder. Add the melted chocolate, mix well until the chocolate is fully incorporated and add the Armangac to taste. Remove the pastry cream from the fridge and beat into the chocolate mix until smooth. Refrigerate for 1 hour to firm up. Roll into 60g balls, place on a tray and freeze until ready to use.
Divide the puff pastry into two pieces: one of 300g and the other of 400g. Roll out the smaller piece of pastry to a thickness of 2mm on a lightly floured surface. Use a 9cm pastry cutter to cut out 6 round bases for the Pithiviers. Place the bases on a tray and refrigerate while preparing the tops. Roll out the larger piece of pastry in the same way, but to a thickness of 3mm. Cut out 6 tops with an 8cm pastry cutter. Place the tops on the same tray as the bases and rest the pastry for 30 minutes in the fridge before assembling the Pithiviers.
Assemble the Pithiviers one at a time to prevent the pastry from becoming too warm. Place a pastry base onto a lightly floured surface. Put a ball of the chocolate filling into the centre of the pastry base and brush the edge of the base with egg-wash. Place a pastry top over the chocolate filling and mould it around the chocolate, removing any air pockets between the filling and pastry. Press together the two pieces of pastry so that they are firmly joined. The Pithivier should be dome shaped with a 1cm border. Place the 9cm pastry cutter over the base of the Pithivier and cut away any excess pastry. Brush with egg wash and refrigerate while preparing the remaining Pithiviers. When all 6 Pithiviers have been assembled, rest for 30 minutes in the fridge before marking.
Mark the border of the pastry at even intervals with the back of a knife. Mark the dome of the Pithiviers at even intervals with the tip of the knife. Start each line from the centre of the dome and curve it down to the base. Take care not to cut too deeply into the pastry. Lightly spray a baking try with cold water and place the Pithiviers onto it. Refrigerate for 30 minutes before baking. Pre-heat the oven to 220°C.
Bake the Pithiviers for 8 - 10 minutes until crisp and golden. Remove the Pithiviers from the oven. Increase the oven temperature to 240°C. Dust the Pithiviers with icing sugar and return to the oven for a further 1 - 2 minutes until golden and glazed. Place the chocolate sauce into a plastic squeeze bottle.
Place a Pithivier onto a plate with a chocolate tuille beside it. Set a scoop of prune and Armangac ice cream onto the tuille. Squeeze a line of chocolate sauce onto the plate.