In a small saucepan combine sugar, corn syrup, water and salt. Heat mixture over moderately high heat, whisking occasionally, until mixture reaches 240 degrees on a candy thermometer. Meanwhile, in the bowl of an electric stand mixer fitted with whisk attachment, whip together egg whites and cream of tartar on medium speed (I had to whip mine on high speed because the whisk attachment on my mixer doesn't reach quite that low, so if your doesn't either you may want to crank up the speed), until soft peaks form.
Once sugar mixture reaches 240 degrees, remove from heat, set stand mixer containing egg white mixture on low speed and carefully pour about 2 Tbsp of hot sugar mixture down the side of the bowl into the egg white mixture. With mixer running, slowly pour remaining syrup down the side of the bowl into the egg white mixture, increase speed to medium-high and whip mixture until stiff glossy peaks form, about 8 - 10 minutes. Stir in vanilla during the last minute of mixing. Use immediately or store in an airtight container in refrigerator up to two weeks.
Recipe Source: adapted from chow who adapted it from "Marshmallow Madness!" by Shauna Server.