Place coffee and rum in a saucepan and heat to simmer. Add the chocolate and the butter or margarine. Cook, stirring occasionally, until both are melted. Remove from the heat.
In a large bowl beat the eggs. Gradually beat in the sugar until the mixture is thick. Beat in the vanilla and the chocolate mixture. In another bowl stir together the flour, bicarb soda and salt. Beat into the chocolate mixture.
Turn the batter into a greased and floured ring tin. Bake in a preheated 135 degrees C oven for 1 1/2 hours, or until the cake tests done with a toothpick. Transfer to a rack to cool. Makes 16 servings.