1. grease 2 deep, 24 cm round cake pans, line bases and sides with baking paper.
2. combine butter, water, cocoa, sugar and baking soda in a LARGE saucepan. stir over a low heat until butter is melted. bring to a boil and simmer uncovered 2 minutes.
3. remove from heat and cool 5 minutes.
4. whisk in flour.
5. add sour cream and eggs and mix until well combined.
6. divide mixture evenly between pans.
7. cook cakes in moderate oven (180 c) for about 30 mins or until cooked.
8. stand in pans for 5 mins then turn onto wire racks to cool.
9. to make chocolate ganache combine chocolate, thickened cream and butter in a small pan.
10. stir over a low heat until mixture is smooth.
11. remove from heat and transfer to a bowl, refrigerate until mixture is a spreadable consistancy.
12. spread ganache over top and sides of one cake, refrigerate until set.
13. place un-iced cake on serving plate and top with whipped cream, placed iced cake on top of cream and dust with cocoa and icing sugar to decorate.