This recipe is adapted from the Australian Women's Weekly "Decorating Cakes" cookbook.
Because this is cooked on a low heat and is a deep dish cake it cooks for approximately an hour. If you were cooking it in 2 tiny cake tins it would cook for far less time
Preheat your oven to 160ºC / 325ºF ( yes this is a low oven temperature but it cooks for a long time).
Grease and line a Deep 15cm (6 inch) cake tin.
Combine broken chocolate and sifted cocoa and add to the water in a saucepan on the stove. Stir over a low heat until the mixture is smooth then transfer it to a bowl and cool for 15 minutes.
To this bowl of chocolate mixture add butter, sugar, eggs, vanilla extract, sour cream ad the sifted dry ingredients.
Beat all the ingredients on low speed until combined and then increase the speed to medium. Beat it for 3 minutes or until the mixture is smooth and changed to a paler colour.
Spread into the prepared tin and bake for 1hr to 1 1/4hrs in a thermofan oven. If you are not using a thermofan oven this can cook much quicker so you will have to watch it.
Test cake by inserting a skewer into the centre of the cake. If it is cooked the skewer will come away clean. If there is any mixture on the skeyer, bake it for a further 10 minutes before testing.
Allow the cake to cool in the pan, and when cool turn out onto a wire cooling rack.
Once cool I like to place the cake in the fridge (wrapped in plastic wrap so that it doesn't absorb any of the fridge smells). Once it is cold and firm I then slice it into 3 even layers before icing with your favourite icing.
Decorate with flowers, or chocolate curls.