Preheat the oven to 170°C (150°C Fan, Gas Mark 3). Grease and line a 900g (2 lb) loaf tin. Put the butter in a saucepan with the sugar, prunes and treacle and place over a very low heat, stirring occasionally, until melted together. Remove from the heat and stir in the milk. Leave to cool for 10 minutes.
Sift the flour, Cocoa Powder and Bicarbonate of Soda into a bowl and make a well in the centre. Add the egg and Chocolate Chunks. Gradually pour in the melted mixture, carefully mixing together to make a smooth, thick batter.
Bake in the oven for about 50 minutes until risen and firm to the touch, and a skewer inserted into the centre comes out clean. Cool for 10 minutes then turn on to a wire rack to cool completely – the loaf will sink to a flattish top as it cools. Wrap and store for at least 24 hours before serving to allow the flavour and texture to develop.
To decorate, break the Dark Chocolate into pieces and place in a small heatproof bowl over a saucepan of barely simmering water until melted. Remove from the water and scrape into a piping bag fitted with a fine plain nozzle. Pipe zig-zag stripes on top of the cake. Leave for a few minutes to set before slicing to serve.