Preheat the oven to 340 degrees F and butter the cake pan.

Melt the butter and chocolate over hot (Not boiling) water in a double boiler. Add the coffee and whip well with a wire whisk. Remove from the heat and gradually whip in the flour, sugar, and slat. Whip in the rum. Mix in the egg yolks.

Beat the egg whites until stiff but not dry, and fold into the cooled chocolate mixture. Place in a water bath and bake for 1 hour and 10 minutes. Cake is done when a toothpick inserted in the center comes out clean.