Put the water with the sugar and cook until it reaches 115°c. In the mean-time whip the egg yolks until pale and creamy; gradually pour in the hot syrup whisking continuously until cool. Chop the candied walnuts roughly and add to the yolk mixture, fold in white choc, the syrup and the crushed meringues. Whip the cream and fold in the yolk mixture, divide into desired moulds and freeze for at least 2 hours. To make the sauce, whisk together cocoa powder, sugar and water. Add the cream and bring to the boil. Serve the semi freddo accompanied with the choc sauce and garnish with few walnuts and crushed meringues.