Makes about 8 cups
Combine 1 1/4 cups sugar with 2/3 cup water in a medium saucepan and bring to a boil over medium-high heat. Attach a candy thermometer to the side of the saucepan and cook without stirring until syrup reaches 235 degrees.
Meanwhile, in the bowl of a standing mixer with the whisk attached, whip egg whites to a soft peak on medium speed. Add remaining 1/4 cup of sugar, cream of tartar, vanilla extract, and salt and continue to whip for 1 minute. Very carefully, in a slow, steady stream, pour hot sugar syrup down the side of the mixing bowl while mixer is running. Continue to whip frosting for seven minutes or until frosting has cooled. Use immediately.
Cook's Note: It is important to use the icing as soon as possible because it will become stiff and more difficult to work with as it sits.