Preheat the oven to 160°C/325°F/gas mark 3. Line the tin with greaseproof paper or baking parchment.
Cut the butter into cubes and place in a heavy saucepan along with the chocolate, sugar, treacle and buttermilk. Heat gently until the ingredients have melted, then set aside to cool.
Snip the prunes into small pieces with kitchen scissors. Sift the ﬂour into a large bowl along with the bicarbonate of soda and spices. Pour the chocolate mixture into the bowl and beat thoroughly with a wooden spoon, then add the beaten egg and beat again. Fold in the prunes.
Pour the mixture into the prepared tin and level off the surface using a palette knife. Bake for about 50 minutes. Remove from the oven and leave to cool in the tin for about 10 minutes. Turn out on to a wire rack and leave to cool completely. Wrap in grease-proof paper and store in an airtight container.
This cake is wonderfully moist and will keep for a week in an airtight container. It is best eaten the day after it is made.
The time quoted above is an estimate of the time needed until the recipe is ready to eat, allowing for preparation, baking and resting.
As you might expect from Green & Black's, this recipe is indulgent and should be enjoyed as a treat..