Melt the butter in a medium sized pan over a low heat.
Dissolve the coffee in the water and add to the butter.
Break up the chocolate add to the pan and turn off the heat. Allow to melt then stir until smooth.
Add the cardamom oil or ground cardamom and stir again.
Beat the sugar, eggs and salt together with electric beaters on medium speed for six to ten minutes or until the mixture has tripled in volume and is thick and mousse like.
Pour the chocolate mixture down one side of the bowl and fold in as gently as possible until just incorporated.
Pour into a deep 23cm round silicone mould or lined cake tin.
Bake at 160℃ (320℉, Gas 3) for about an hour or until a skewer inserted into the cake comes out almost clean.
Allow to cool, then turn the cake out onto a serving plate and dust with the cocoa powder.
Don't worry if the cake sinks and cracks, this is normal.Adapted from Ottolenghi: The CookbookPlease note: calories and other nutritional information are per serving. They're approximate and will depend on exact ingredients used.