Combine the sugar, corn syrup, water and salt in a small saucepan over medium-high heat. Bring to a boil. Stirring occasionally, continue boiling until the mixture reaches 240° F on a candy thermometer.
While the sugar mixture is boiling, add the egg whites and cream of tartar to a stand mixer bowl fitted with a whisk attachment. Whip on medium-high speed until soft peaks form. The egg whites should be ready and waiting when the sugar mixture reaches 240 degrees F. Turn off the mixer once the egg whites are ready.
Turn the mixer speed to low and very slowly drizzle the boiled syrup down the side of the mixer bowl. Once all of the syrup has been added, increase the speed to medium-high and beat until stiff, glossy peaks form. The mixture should have cooled substantially; this may take around 7 to 9 minutes. Add the vanilla extract during the last minute or two.
Use immediately, or store in an airtight container in the refrigerator for up to 2 weeks.