In a small saucepan, combine sugar, corn syrup, and the water. Bring to a boil over high heat, stirring to dissolve the sugar. When sugar has dissolved, continue to cook without stirring, until it reaches 248 degrees as measured on a candy thermometer.
In the bowl of an electric mixer fitted with the whisk attachment, beat egg whites, vanilla, and salt until stiff peaks form. When sugar mixture has reached 248 degrees. pour it into the beaten egg whites while beating constantly on high speed, until the mixture is cool and is the proper consistency for spreading, about 7 minutes.