Preheat oven to 140C/280F. Grease two deep 20cm (8 inch) round cake pans; line bases and sides with baking paper.
Stir chocolate, sifted cocoa and the water in a medium saucepan over low heat until smooth. Transfer mixture to a large bowl; cool for 15 minutes.
Add butter, sugar, eggs, vanilla, sour cream and sifted dry ingredients to chocolate mixture; beat on low speed with an electric mixer until combined. Increase speed to medium; beat for about 3 minutes or until mixture is smooth and paler in colour. Divide the mixture evenly between pans.
Bake cakes for about 1 1/4 hours. Stand in the pans for 5 minutes, then transfer, top-side up, to wire racks to cool.
Meanwhile, make the ganache. Stir ganache ingredients in a large heatproof bowl over a large saucepan of simmering water until smooth. Cover; refrigerate for 3 hours or until thick. Beat ganache in a large bowl with an electric mixer until firm peaks form.
Split cakes in half. Sandwich cakes using 1/2 cup of the ganache between each layer. Position cake on a cake stand or a prepared cake board; secure cake to stand or board with a little ganache. Spread cake all over with the remaining ganache.