1.Start making rum and raisin filling. Combine rum and raisins in a small bowl. Cover; stand 3 hours.
2.Preheat oven to 180°C/350°F. Grease a 20cm x 30cm lamington pan; line base with baking paper, extending paper 5cm above two long sides.
3.Stir butter, chocolate, sugar and the water in a medium saucepan over low heat until smooth. Remove from heat; stir in sifted flour and cocoa, then egg yolks. Pour mixture into pan; bake about 15 minutes. Cool in pan.
4.Meanwhile, continue making filling. Melt the chocolate; cool slightly. Beat cream cheese and sugar in a medium bowl with an electric mixer until smooth. Beat in eggs, one at a time. Stir in chocolate, then the rum and raisin mixture. Pour filling over base.
5.Bake cheesecake for 45 minutes or until just set. Cool in oven with door ajar. Refrigerate for 3 hours or overnight. Cut into squares; serve cheesecake dusted with extra cocoa.