Preheat oven to 180 degrees C. Line the bottom of a 23cm round cake tin with baking paper.
Bring butter, coffee and sugar to a boil in a medium saucepan, stirring to dissolve sugar.
Place chocolate in a large bowl and add boiling hot coffee mixture. Whisk until smooth. Cool slightly. Whisk in eggs. Pour batter into prepared pan.
Place cake pan in a baking tray. Pour enough hot water into the baking tray to come half way up the sides of the cake pan. Bake until centre of cake is set, and a skewer inserted comes out clean with a few moist crumbs attached, about 1 hour. Remove from water. Chill overnight.