In the bowl of an electric mixer fitted with the whisk attachment, beat egg whites and cream of tartar together until light and frothy. With the mixer running, slowly pour in 2 tablespoons sugar; beat until soft peaks form. Set aside.
In a small saucepan, combine 1/3 cup water, corn syrup, and remaining 2/3 cup sugar. Place over medium heat and cook until boiling.
Cook, stirring, until mixture reaches the soft-ball stage, about 240 degrees on a candy thermometer, about 6 minutes. Immediately remove from heat.
With the mixer on low, slowly add hot syrup to egg-white mixture. Increase mixer speed to high and continue beating for 5 minutes.
Add vanilla and continue to beat until mixture looks like marshmallow cream, about 1 minute more.
Allow to cool, then spoon into an air-tight container. Can be stored in the fridge for up to a month.
This is so much tastier than store bought!