Recipe from Kitchn

Makes 2 to 3 cups

9 oz water (1/4 cup plus 2 tablespoons)
3/4 cup granulated sugar
3/4 cup light corn syrup
4 liquid ounces egg whites (from about 3 large eggs)
1/2 teaspoon cream of tartar
Pinch salt
1 1/2 teaspoons pure vanilla extract

Instructions

Place the water, sugar, and corn syrup in a medium saucepan. Warm over medium-high heat until the mixture reaches 240°F on a candy thermometer, typically 10 to 15 minutes.

 

While the sugar is heating, beat the egg whites to soft peaks so they’re ready at the same time as the sugar. Place the egg whites (4 fl oz) in the bowl of an electric mixer fitted with a whisk attachment. Beat on medium until the egg whites are foamy. Increase the speed to high, and add in the cream of tartar (0.5 tsp) and salt (pinch). Continue to mix until soft peaks form.

Once the sugar mixtures reaches 240°F, carefully remove it from the heat and let rest for about 20 seconds. With the mixer running on high, slowly stream the sugar mixture into the egg whites. Continue to mix for 7 to 8 minutes, or until the outside of the mixing bowl returns to room temperature. Add in the vanilla (1.5 tsp); mix until medium-stiff peaks form.

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

 

I have to put in a plug for this OXO measuring cup.  It is perfect for measuring sticky, gooey things like marshmallow fluff.  

Recipe Notes

Keep a close eye on your candy thermometer. Typically, the sugar syrup gets “stuck” around 200 to 220°F, but picks back up again rather quickly. This whole process should take about 10 to 15 minutes.

Be sure to start mixing your egg whites before the sugar reaches 240°F.

If you start whipping your eggs whites too early, it is okay to stop the mixer while you wait for your sugar syrup to reach its proper temperature.

When pouring in the sugar syrup into the egg whites, try to avid hitting the whisk and aim for the edge of the bowl instead.

For a more intense vanilla flavor, use 1 1/2 teaspoons vanilla bean paste or the seeds of a vanilla bean pod.

As with honey and molasses, spray the inside of your measuring cup with non-stick spray before measuring the corn syrup — it will come out much easier!