You can do this step well in advance, otherwise, make it the night before. Place the Armagnac in a pan with the spices, finely grate in the orange zest and bring to the boil. Pop the prunes in a bowl, pour over the spiced Armagnac, cover with clingfilm and place in the fridge.
When ready to make the cake, line a 20cm cake tin and preheat your oven to 180°C/350°F/gas 4.
Break up the chocolate into pieces and put with the butter in a bowl over a pan of simmering water. Stir occasionally, until melted, then take off the heat.
Separate the eggs. In a spotlessly clean bowl, whip the egg whites with 1 tablespoon of the sugar until it forms hard peaks. Place the yolks and remaining sugar in another bowl and whisk for about 5 minutes, until you can draw a figure of 8 in it and it retains its shape.
Pour the melted chocolate into the whisked yolks and stir to incorporate. Roughly purée the prunes in a food processor, then fold them into the mixture.
Combine the flour and ground almonds in a bowl, then fold into the chocolate mixture. Carefully fold in the egg whites, in three stages, so you don’t lose any of the air you have whisked into it.
Once everything is thoroughly mixed in, pour the mixture into the tin and bake for about 1 hour, until a skewer inserted in the centre comes out clean.
Leave the cake to cool slightly before turning out onto a plate and dusting with cocoa powder. Delicious served with more hot Armagnac-soaked prunes on the side.