Combine sugar and water in saucepan and bring to a boil. Cover and let boil 3-5 minute until sugar dissolves, uncover and continue boiling until syrup reaches 238° on a candy thermometer (soft-ball stage).
Meanwhile, place egg whites in the bowl of a standing mixer fitted with the whisk attachment, and beat on low speed until foamy. Add cream of tartar and salt, and beat on medium-high speed until stiff but not dry; do not overbeat.
When syrup reaches 240 degrees pour 1/4 cup of syrup into egg white mixture. With mixer running, add syrup to whites in a stream, beating on high speed until no longer steaming, about 3 minutes. Add rest of syrup slowly and beat for 8-12 min., or till cool, so butter will not melt.
In seperate bowl cream butter. Add butter bit by bit, beating until spreadable, 3 to 5 minutes; beat in vanilla and Grand Marnier. If icing curdles, keep beating until smooth, may still be too warm. Use Immediately!