Preheat oven to 180°C. Grease a 24cm x 8.5cm (12 cup capacity) fluted tube tin with butter. Put water, sugar and butter in a large saucepan over a medium heat and stir until butter has melted and mixture is smooth.
Reduce heat to medium-low. Sift cocoa and bicarb on top of butter mixture. Whisk until combined. Bring to a gentle simmer for 1 minute. Allow to cool slightly. Add eggs, coffee, and flour and whisk until smooth.
Pour into tin and bake for 45-50 minutes, or until a skewer comes out clean when inserted into cake. Allow to cool for 30 minutes, then invert onto a wire rack to cool.
Melt chocolate in a heatproof jug in the microwave on 50 per cent (medium) for 1-1½ minutes. Stir until melted. Microwave for a further 30 seconds on 50 per cent if required. Allow to stand for 15 minutes, then pour over cake. Allow chocolate to set, then serve with whipped cream, if desired.