First place 350g of the prunes in a medium-sized saucepan and add 425ml of cold water. Bring this up to simmering point, cover and simmer gently for 20 minutes, then leave to cool.
Now whip the double cream until it reaches the floppy stage but isn't too thick then pop it into the fridge to chill.
Next make the custard - first pour the single cream into a medium saucepan, then carefully heat it to just below boiling point. Meanwhile, beat together the egg yolks, cornflour and caster sugar in a bowl until absolutely smooth.
Next pour the hot cream on to this mixture, whisking as you pour. Now return the custard to the pan and continue to whisk over a medium heat until it has thickened and come to just below boiling point again (don't worry, it won't curdle - if it does look at all grainy, it will become smooth again when whisked off the heat).
Rinse out the bowl and pour the custard into it.
Then place it in another larger bowl of cold water with a few ice cubes, stirring it now and then until it is absolutely cold.
While the custard is cooling, drain off the cooking liquid from the prunes and puree them in a food processor with the Armagnac until very smooth. Now roughly chop the remaining prunes into 5mm pieces.
When the custard is completely cold, stir in the prune and Armagnac puree, then fold in the chilled whipping cream.
Now pour the mixture into the ice-cream maker and freeze-churn according to the manufacturer's instructions (you may have to do this in two batches and it will take between 30-60 minutes depending on the capacity of your machine).
When the ice cream is soft-set, add the diced prune and churn it until all in incorporated then transfer to a plastic box and freeze for 2 hours before you serve.
If the ice cream is made well in advance and has frozen solid, remove from the freezer for 30 minutes before serving to soften.
NOTE: If you don't have an ice cream maker you can still make ice cream. After you have made up your mixture, transfer it to a lidded plastic box and put it in the coldest part of the freezer for two hours or until the contents become firm at the edges.
At this stage, empty out the box into a mixing bowl and whisk the ice cream with an electric hand whisk to break down the ice crystals. Return to the plastic box and freeze for another two hours, then repeat the whisking process.
Refreeze the ice cream until 30 minutes before you want to serve it.