Set the oven at 180C/gas mark 4.Line a 21cm square baking tin with baking parchment.
Cut each prune in half and put in a bowl. Pour over the armagnac and leave for 30 minutes.
Put the caster sugar and butter in the bowl of a food mixer and beat thoroughly till pale and creamy. Snap the chocolate into pieces and put them in a small heatproof bowl balancing over a small pan of simmering water.
Leave the chocolate to melt, avoiding the temptation to stir. Take care the chocolate doesn’t get too hot.
As soon as the chocolate is liquid, turn off the heat. Break the eggs into a bowl and add the extra yolk. Beat them lightly with a fork.
Sift the flour, cocoa and baking powder together and set aside.
Add the beaten egg to the butter and sugar mixture, then, with the mixer on low speed, pour in the melted chocolate. Stop the machine.
Add the chopped prunes, drained of any excess liquid, and then fold in the dry ingredients. Take care not to over-mix, or your brownie will be flat and heavy.
Scrape the mixture into the prepared cake tin and bake for 30 minutes. Remove from the oven and leave to cool before slicing into (at least) 12.