Start by making the meringues. Preheat the oven to 100°C/gas mark 1/4. Line two baking trays with baking parchment. 

Whisk the egg whites to the soft peak stage and add the caster sugar, a spoonful at a time, continuing to whisk until the mixture is thick, very glossy and smooth (you shouldn’t be able to feel grains of sugar if you rub a little of the mixture between your fingertips). Fold in the icing sugar then add the cocoa and fold until the chocolate brown just ripples through the white. Spoon or pipe the mixture into meringues about 4cm in diameter on the parchment paper. Bake for 1 hour, or until firm, crisp and easy to lift from the parchment. Cool and store in an airtight tin until needed. 

To make the cake, preheat the oven to 170°C/gas mark 3. Grease and line a 20cm cake tin. 

Start by mixing the prunes and walnuts with a small spoonful of the flour, just to give them a dusty coating. Set aside. Sift the remaining flour and the baking powder into a roomy mixing bowl. Add the butter, sugar and egg and use an electric whisk, mix for about a minute until smooth and creamy. Use a metal spoon to fold the prunes and nuts into the mixture and spread the mixture into the cake tin. Bake for 25 minutes, or until the cake is springy to the touch. Turn onto a rack and leave to cool completely before assembling the cake. 

To make the cream, beat the two creams together until they just form stiff peaks. Spread the top and sides of the cake with some of the cream. Roughly crumble 4 or 5 of the meringues into the remaining cream and fold together. Pile this mixture in a mound on top of the cake. Stick the remaining meringues into the cream to cover the surface. 

For a final decorative flourish, melt the chocolate in a heatproof bowl over a pan of simmering water. Leave to cool then drizzle over the top of the cake.