Pre heat oven 160o C for 1 hour
For a 16 cm round tin or 14 cm square tin line the bottom and sides with baking paper
Sift flour, cocoa powder and bicarbonate soda together into a large bowl. Add the caster sugar to the flour mixture and stir until well combined. Use a whisk to do this.
Place egg, buttermilk, vanilla extract in a bowl and whisk until well combined.
Put chocolate, butter coffee and water in a saucepan over low heat, stirring occasionally. When chocolate and butter has melted remove from the heat and allow to cool to room temperature.
Add the wet ingredients to the flour mixture and stir until well combined.
Add the chocolate mixture to the egg and flour mixture in 3 batches, stirring until combined after each addition.
Bake for 1 hour or when a clean skewer is inserted comes out clean.
Place chocolate and cream in a medium pan over a low heat. Stir frequently and remove from heat when chocolate has melted and mixture is smooth. If necessary allow the ganache to cool at room temperature to obtain a spreadable consistency.