Bring granulated sugar and 2/3 cup water to a boil in a small saucepan. Swirl pan to dissolve sugar. Wash down sides of pan with a wet pastry brush to prevent crystals from forming. Boil until syrup registers 238 degrees on a candy thermometer.Advertisement
Meanwhile, put egg whites in the bowl of an electric mixer fitted with the whisk attachment, and beat on low speed until foamy. Raise speed to medium-high. Add cream of tartar; mix until stiff but not dry.
With mixer running, slowly pour in hot sugar syrup, and mix on high speed until mixture has partially cooled, 5 to 7 minutes. Reduce speed to medium, and slowly add confectioners' sugar. Raise speed to medium-high, and mix until mixture has cooled completely, 7 to 10 minutes more. Use immediately.