Heat the oven to 350°F/175°C. Grease and flour a 9-inch/23 -cm springform pan, wrapped with foil around the outside bottom to prevent leakage. Put the chocolate and coffee in the top of a double boiler and heat to melt. Stir until smooth and add the butter, a few chunks at a time, until the mixture is smooth and glossy. Beat in the sugar, and cook the mixture a minute or two until hot.
Beat the eggs in a bowl. Slowly pour over the chocolate mixture, whisking constantly. Pour the batter through a strainer into the pan. Bake until the cake is cooked through, about 40 minutes. Take it from the oven and cool in the pan 5 minutes, then run a knife around the edges and remove the sides of the pan. Cool completely. Wrap the cake very well in plastic and refrigerate overnight or up to two days. Take the cake from the fridge, unwrap it, and set it on plate at room temperature about an hour before serving. This is good plain, or with raspberries and whipped cream, or with custard sauce…