Preheat oven to 160 C (not fan forced)
Grease side and base of a 23 cm round springform cake tin for easy removal of cake. Line base and side of the pan with baking paper, extending the paper a few centimetres above the top rim of the pan.
Place 250 grams of chocolate , butter, coffee and water in a large saucepan over low heat, stirring occasionally. When chocolate and butter have melted, remove mixture from heat and allow to cool to lukewarm/room temperature
Sift the flour , cocoa powder and baking soda together into a very large bowl. Add the caster sugar to the bowl with the flour mixture, and whisk the ingredients together until well combined.
In a medium bowl, place eggs, oil, vanilla and buttermilk. Whisk the wet ingredients together until well combined. Add the egg mixture to the flour mixture in three batches, stirring until combined after each addition.
Pour mixture into prepared pan,
Bake for about 1 hour 30 minutes. Insert a thin bladed knife or wooden skewer into the centre of the cake to test whether it is cooked through. If the knife/skewer come out clean or with moist crumbs (not gooey batter) attached, the cake is ready. Remove cake from oven , cover with a clean tea towel and allow to cool in the pan.
When cake is cooled to room temperature , remove it from the pan. The top surface of the cake will probably have a crunchy crust, which you may want to trim off. Turn cake upside down to give a flat surface for icing.