Warm Sherry over low heat, add prunes and set aside to soak for 30 minutes, then drain, reserving Sherry and prunes separately.
Preheat oven to 180C. Process walnuts in a food processor until finely chopped. Heat butter, chocolate and sugar in a saucepan over low heat until smooth, then set aside to cool. Add egg yolks and walnuts, and mix well.
Whisk eggwhites and ½ tsp salt in an electric mixer to soft peaks (2-3 minutes). Fold a third of the eggwhite into chocolate mixture, then add remaining eggwhite and prunes and fold to combine. Spoon into a buttered 23cm-diameter cake tin lined with baking paper and bake until a skewer inserted withdraws clean (35-40 minutes). Cool in tin for 10 minutes then turn out onto a wire rack to cool completely (1-2 hours).
For chocolate and Sherry ganache, stir chocolate, cream and reserved Sherry in a small saucepan over low heat until chocolate melts, then set aside to cool (15 minutes). Spread ganache over cooled cake, scatter with grated chocolate and serve. Before icing, the cake will keep in an airtight container for 4-5 days.