This icing looks beautiful and is a buttery treat on top of cakes such as Raspberry Ruffle.
In a small saucepan over medium heat, bring sugar and water to a boil. Continue to boil until syrup reaches soft-ball stage, 238 degrees on a candy thermometer.
Meanwhile, beat egg whites on low speed until foamy. Add cream of tartar, and beat on medium high until stiff but not dry.
With mixer running, pour syrup into egg whites in a steady stream and beat on high speed until steam is no longer visible, about 3 minutes. Add butter, bit by bit. After all the butter has been added, beat for 3 to 5 minutes, until smooth and spreadable. If the icing looks curdled at any point during the beating process, don't panic: keep beating and it will smooth out.
If icing becomes too soft for piping, stir over ice water to stiffen. If it's been stored in the refrigerator, bring it back to room temperature and rewhip.