Boil the kettle and make a nice big pot of Extra Strong Typhoo tea (if you don't have a teapot, use a jug), leave to soak until you have dark brown tea.
Put the prunes in a bowl and pour over enough tea to cover them, leave overnight or for at least 8 hours until the prunes are plumped up and have absorbed most of the tea.
Drain the prunes, keeping the tea in a bowl to use later in the glaze. If the prunes have stones, remove the stones, then puree the prunes in a food processor or electric blender.
Grease an 8in/200mm ring cake tin, Bundt tin or you can use an ordinary cake tin with a jam jar in the middle.
Beat the sugar and butter together until soft and creamy. Beat in the eggs, one at a time, then add the prune puree.
Mix the dry ingredients together and add to the creamed sugar and butter, with the buttermilk. Beat well.
Finally, add the chopped walnuts and chocolate and fold into the mixture.
Turn the mixture into the cake tin and bake at 350F/180C/Gas 4 for 30 minutes then increase the heat to 375F/190C/Gas 5 and bake for another 25 minutes.
Check it is baked with a skewer which should come out clean, the cake should also be easing away from the sides of the tin.
Cool in the tin for 15 minutes, then turn onto a wire rack to finish cooling.
When the cake is completely cool make the glaze.