1 For the cake: heat the oven to 170°C (150° fan) 325°F gas 3. Grease a 23cm x 12cm| 9" x5" loaf tin and line the base with non-stick baking paper.
2 Put the prunes, water and sherry in a small pan and bring to a boil. Remove from the heat.
3 Melt the chocolate in a heatproof bowl over a pan of simmering (not boiling) water and remove from the heat.
4 Beat together the butter and brown sugar in a mixing bowl, then add the eggs, beating well until blended.
5 Pour in the melted chocolate and mix until just combined. Sift in the flour and baking powder and stir into the mixture, alternately with the hot prune mixture.
6 Put into the tin and bake for about 1 hour. A skewer inserted into the centre should come out with only a few moist crumbs clinging to it. Cool in the tin for 15 minutes, then place on a wire rack to cool completely.
7 For the icing: sift the icing sugar into a bowl and gradually beat in enough water to make a smooth thick icing.
8 Spread about 2/3 of the icing over the cake.
9 Sift the cocoa powder into the remaining icing and more hot water if it is too thick. Drizzle lines over the top of the white icing and leave to set.