DIY Marshmallow Creme
- 3 egg whites
- 1/2 teaspoon cream of tartar
- 1/3 cup water
- 5 1/2 ounces granulated sugar, divided (3/4 cup plus 2 tablespoons)
- 9 ounces light corn syrup (3/4 cup)
- 1 teaspoon vanilla extract
Place the egg whites in the bowl of a stand mixer and sprinkle the cream of tartar over. Beat, increasing the speed to medium-high, until the eggs starts to get light, airy, and frothy. With the mixer running, slowly pour in 7/8 ounce (2 tablespoons) granulated sugar. Continue to beat until firm (but not stiff) peaks form. Set aside.
Fit a small saucepan with a candy thermometer, or have a reliable digital thermometer ready nearby. Combine water, sugar, and corn syrup in the saucepan and stir together. Bring to a boil over medium heat, and then, using a heat-safe spatula, stir very frequently as you cook the syrup mixture to 240°F, maintaining a consistent boil. Once at 240°F, remove from the heat immediately.
Start the mixer on second speed and slowly pour the syrup in as it mixes until all of the syrup has been added. Stop the mixer and scrape down the bowl if needed, and then increase speed to medium-high and beat for another 5 minutes. The mixture should have expanded and you should now have a white creme that's able to hold some shape. Add the vanilla, wipe down the bowl and beater, and beat for about 1 more minute, until the mixture is even in color again and has reached the desired consistency of marshmallow creme.
Store creme in an airtight container in the refrigerator.