Put a kettle on to boil. Heat the oven to 160°C (325°F, gas mark 3). Grease a 23 cm (9 in) square cake tin and line it with baking paper.
Purée the prunes in a food processor or with a hand-held mixer, adding 1 tablespoon of hot water if necessary to get a smooth consistency. Scrape the purée into a large mixing bowl.
Add the brandy, butter, chocolate, sugar and 250 ml (9 fl oz) of boiling water from the kettle. Put the rest of the boiling water into a saucepan over a low heat so that it is simmering and place the bowl on top, making sure the bottom does not touch the water.
Stir the mixture frequently until the chocolate melts and forms a smooth sauce. Take the bowl off the pan and leave the mixture to cool for 2-3 minutes until it is lukewarm.
Sift both flours and the cocoa powder into a large bowl and make a well in the centre. Pour in the chocolate mixture, then add the whole egg and egg whites and beat to a smooth batter.
Pour the mixture into the prepared tin and smooth the surface. Bake for 1 hour 15 minutes, or until a skewer inserted in the centre of the cake comes out clean.
Leave the cake to stand in the tin on a wire rack for I5 minutes, then turn it out onto the wire rack to cool completely.
Dust the cake with sifted icing sugar or cocoa powder and cut it into 16 squares. Serve plain or with low-fat yoghurt, iced dessert or cream, if you prefer.
Variation: for a sweeter, fruitier flavour, use port instead of brandy.
I love this recipe, and in this variation I saved time by doing steps 2-4 in the food blender. I used Lindt 70% chocolate (2 bars) and broke it into squares, then used the blender on ice cube setting. The whole mixture came out beautifully. I cooked the mix for the recommended time, and noticed that cracks appeared in the top so I melted the remaining half bar of choc with a litlle butter and just filled in the cracks :-) Gorgeous! - 26 Dec 2010