Add the water, sugar, and corn syrup to a medium-sized saucepan. Heat over medium heat while stirring until the sugar is completely dissolved.
1/2 cup water, 3/4 cup granulated sugar, 3/4 cup light corn syrup
Stop stirring and bring the mixture to a boil. Cook until the sugar mixture is 240F.
Meanwhile, put the egg whites and cream of tartar in a bowl, and use an electric mixer on high to beat them. Beat the egg whites until they form soft peaks, which should take 3-4 minutes.
2 large egg whites, 1/2 teaspoon cream of tartar
When the sugar mixture is up to temperature, slowly pour it into the egg white mixture while beating on low.
After all the sugar has been added, turn the mixer to high, and beat until the marshmallow fluff is glossy and thick. This should take about 7-8 minutes.
Beat in the vanilla extract, then serve or store.
1 teaspoon vanilla extract
Notes
Leftover marshmallow fluff can be stored in an airtight container at room temperature for 2 weeks, in the fridge for 2 months, or in the freezer for 6 months.
Be very careful when boiling the sugar mixture, as boiling sugar can cause serious damage if it comes into contact with your skin.