In top of double boiler, combine sugar, water, egg whites, corn syrup and salt. Beat with electric mixer on low speed for 1 minute. Place over boiling water (upper pan should not touch water) and beat on high speed until stiff peaks form, about 7 minutes. Remove from heat; add vanilla. Beat 2-3 minutes more or until easy to spread.
Makes about 5 1/2 cups.
Quick Tip
When preparing Seven Minute Frosting, beat ingredients over boiling water, using a portable electric mixer, to dissolve the sugar so frosting won't be grainy.