This buttercream holds its shape particularly well, making it good for decorating.
In a small saucepan over medium heat, bring sugar and 1/3 cup water to a boil. Boil until syrup reaches soft-ball stage (238 degrees on a candy thermometer).
In the bowl of an electric mixer, use the whisk attachment to beat egg whites on low speed until foamy. Add cream of tartar and beat on medium high until stiff but not dry.
With the mixer running, pour the sugar syrup into egg whites in a steady stream and beat on high speed until steam is no longer visible, about 3 minutes. Beat in butter, piece by piece. Add vanilla and beat for 3 to 5 minutes, until frosting is smooth and spreadable. If it looks curdled at any point during the beating process, keep beating to smooth it out. If it becomes too soft for piping, stir over ice water to stiffen.