Break up the dark chocolate and let it melt slowly over a hot but not boiling bain-marie; remove it from the heat and mix in the cream, stirring until the mixture is even. Let cool. Place a small dollop of the ganache on 18 halved walnuts, then top with the remaining walnut halves and press together gently. Spread the walnuts out on a tray and freeze for about 30 minutes. Melt the white chocolate over a bain-marie; let it cool, then use it to coat the walnuts once they have chilled. Coat the walnuts in plenty of cocoa powder so they are fully covered. As the walnuts were chilled in the freezer, the white chocolate will set quickly. Spread the walnuts out on a tray and keep them cool until ready to serve.