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1
Place the egg whites in the bowl of a standing mixer fitted with a whisk attachment and mix them on low speed until they form soft peaks.
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2
While the eggs are beating, fasten a candy thermometer to the side of a medium saucepan.
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3
Combine the sugar, corn syrup, and 1/2 cup of water in the saucepan and cook over high heat, brushing down the sides of the pot to remove the sugar crystals, for 5 to 8 minutes, until the temperature reads 231F.
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4
Increase the mixer speed to high and continue to whip the whites until they form stiff peaks.
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5
Turn off the mixer until the sugar reaches 234F.
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6
Return the mixer to high speed and pour the hot sugar syrup in a thin, steady stream along the sides of the bowl to combine it with the egg whites.
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7
When all of the syrup has been added, continue to beat until soft peaks form.
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8
Do not overwhip, or the marshmallow sauce will be too stiff.
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9
You want the sauce to be a spoonable consistency but not so loose that it is runny.
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10
Add the vanilla and beat to incorporate it.
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11
Transfer the marshmallow sauce to a medium bowl and set it aside to cool to room temperature before serving.
