1.Preheat oven to 140°C/280°F. Grease two deep 20cm (8-inch) round cake pans: line bases and sides with baking paper.
2.Stir chocolate, sifted cocoa and the water in a medium saucepan over low heat until smooth. Transfer mixture to a large bowl; cool 15 minutes.
3.Add butter, sugar, eggs, extract, sour cream and sifted dry ingredients to chocolate mixture; beat on low speed with electric mixer until combined. Increase speed to medium; beat about 3 minutes or until mixture is smooth and changed to a paler colour. Spread mixture evenly into pans.
4.Bake cakes about 1¼ hours. Stand in pans 5 minutes; turn, top-side up, onto wire racks to cool.
5.Meanwhile, make milk chocolate ganache. Stir milk chocolate and pouring cream in a large heatproof bowl over a large saucepan of simmering water until smooth. Cover; refrigerate 3 hours or until thick. Beat ganache in a large bowl with an electric mixer until firm peaks form.
6.Split cakes in half; sandwich cakes using about ½ cup of the ganache between each layer. Position cake on a cake stand or prepared cake board (see notes), secure with a little ganache. Spread cake all over with remaining ganache.
Prepared cake boards are available from cake decorating and craft stores. You will need one 25cm (10-inch) round board for this cake. The cake and ganache can be made a day ahead. Store cake in an airtight container. Whip ganache and assemble cake up to an hour before serving. Plain cakes can be frozen for up to three months.