The method might sound complicated, but it's really infallible. Just have patience when beating the ingredients together.
In a small saucepan over medium heat, bring the sugar and water to a boil. Continue to boil until syrup reaches the soft-ball stage, 238 degrees on a candy thermometer.
Meanwhile, beat the egg whites until foamy. Add the cream of tartar and beat until stiff but not dry.
With the mixer running, pour the syrup into the egg whites in a thin stream and beat for 3 minutes. Beat the butter into the whites. Beat for 3 to 5 minutes until smooth and spreadable. If the icing "curdles," don't panic: keep beating and it will combine. If it sits for a while, beat it on low speed to smooth it out.