You will need
Break dark chocolate into individual pieces and melt in microwave with water for half a minute on full, or melt in a glass bowl placed over a saucepan of boiling water on heat.
Add egg yolks, butter, rum, fold in softly whipped cream, leave to set a little.
Break white chocolate into individual pieces and melt with half the whipping cream. Fold in yogurt, remaining cream, softly whipped , sugar and vanilla.
Line a 1.5lb loaf tin with baking paper and put in a layer of dark chocolate using half the mixture. Then all the vanilla mixture. Leave until almost completely set and finish with remaining dark chocolate. Cover with cling film and leave to set for at least 6 hours or overnight.
Slice using a hot knife and serve with a chocolate sauce.