Preheat oven to 375°F. Butter four 4 1/2 inch springform pans. In a food processor, process prunes and water until puréed. In a large, heatproof bowl over barely simmering water (ensuring that the water doesn’t touch the bottom of the bowl), melt the chocolate and butter together, stirring occasionally. Remove from heat and stir well. Stir in the prune purée. In a bowl, whisk together the hazelnuts, tapioca flour and salt. In a separate bowl, whisk together the eggs and sugar until evenly blended. Whisk in the chocolate mixture until smooth, then stir in the hazelnut mixture. Pour the batter into the prepared pans. Bake for 25 minutes or until the cakes are set in the center and the top is dry to the touch. Cool completely on a wire rack completely before removing from the pans.