Combine the sugar, corn syrup, water and salt in a small saucepan over medium-high heat. Boil, stirring occasionally, 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 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°F. If the egg whites reach soft peaks before the sugar mixture reaches its temperature, turn off the mixer.

Once the sugar mixture reaches 240 F, turn the mixer speed to low and very slowly drizzle the 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 and the mixture has cooked substantially, about 7 to 9 minutes. Add the vanilla extract during the last minute or two of beating.

Use immediately, or store in an airtight container in the refrigerator for up to 2 weeks.