Combine the sugar, corn syrup, water and salt in a small saucepan over medium-high heat. Attach a candy thermometer to the side of the pan. Heat the mixture, stirring occasionally, until it reaches 240˚ F on the candy thermometer.
Meanwhile, combine the egg whites and cream of tartar in the bowl of a stand mixer fitted with the whisk attachment. As the temperature of the sugar mixture begins to approach its endpoint, begin whipping the egg whites on medium-high speed until soft peaks form. Once soft peaks have been reached, lower the mixer speed to low.
As soon as the sugar mixture reaches 240˚ F on the candy thermometer, remove the pan from the heat. Pour the syrup in a very slow, steady stream down the edge of the mixing bowl until it has been completely incorporated. (It is important to add the syrup slowly so that you don’t cook the eggs.)
Once all of the syrup has been incorporated, return the mixer speed to medium-high and whip until stiff glossy peaks form and the mixture has cooled considerably, about 6-7 minutes. Blend in the vanilla and beat for 2 minutes more. Use immediately or store in an airtight container in the refrigerator for up to 2 weeks.