In a saucepan melt chocolate; cool completely.
In a second small saucepan heat sugar and water to boiling; attach candy thermometer to pan.
Cook mixture to soft-ball stage (candy thermometer registers 240°F).
In small mixer bowl beat egg yolks with electric mixer till thick and lemon-colored.
Very gradually add hot syrup, beating constantly.
Continue beating till mixture is completely cool.
Beat in butter, 1 tablespoon at a time.
Add chocolate, rum, and coffee.
Beat till thick.