1.Preheat oven to 180°C. Grease 24cm closed springform pan or a deep 23cm cake pan.
2.Break chocolate into a medium saucepan, add butter, sugar and the water; stir over low heat until smooth. Transfer to a large bowl; cool 5 minutes. Stir in sifted flour and cocoa,egg yolks, coconut and rum until combined.
3.Beat egg whites in a small bowl with an electric mixer until soft peaks form; fold into chocolate mixture, in two batches. Pour cake mixture into pan.
4.Bake cake about 30 minutes. Cool cake in pan.
5.Make chocolate butter cream
6.Turn cake, top-side up, onto a cake plate; spread butter cream on top of cake. Dust with a little more cocoa and decorate with large chocolate curls, if you like.
Chocolate butter cream
7.Beat butter in a small bowl with electric mixer until light and fluffy. Add sifted icing sugar and cocoa; beat until smooth and fluffy.
The un-iced cake can be made a day ahead, store in an airtight container at room temperature. Decorate with butter cream and chocolate curls on the day of serving.
Dutch-processed cocoa refers to cocoa that has had an alkaline added, during a process called ‘dutching’. The resulting cocoa has a richer chocolate taste and colour.