Preheat oven to 350°. Place pitted prunes (soak in hot water for 30 minutes first, if you have time) in the bowl of a food processor. Add vanilla extract, and process until prunes are broken down into small pieces; set aside. Grease a 10-inch round cake pan. Place chocolate in a heatproof bowl, and microwave (in 1-minute intervals) until melted and smooth. Stir in butter and fold in the prune purée and ground hazelnuts. Beat eggs yolks with sugar with an electric mixer 4 minutes; fold into the chocolate mixture. Beat egg whites with salt with a clean whisk until stiff peaks form. Fold whites into the chocolate mixture in 3 batches. Pour batter into prepared pan, and smooth top. Bake 20 minutes, rotate pan, and continue baking 5–7 minutes. Let cool, cut into 12 slices, and serve.