A warm, drippy marshmallow sauce may be the underdog topping when compared to chocolate sauce, but it adds a little texture and sweet vanilla pop to an ice cream sundae. When refrigerated, its texture stiffens to make the perfect whoopie pie filling, or stick it between two chocolate chip cookies.
Special equipment: A reliable candy/fat thermometer is crucial for getting an accurate read on the syrup.
2 large egg whites, at room temperature
1/4 teaspooncream of tartar
3/4 cuplight corn syrup
1/2 cup granulated sugar
1 1/2 teaspoonsvanilla extract
Place the egg whites in the bowl of a stand mixer fitted with a whisk attachment and beat on medium speed until frothy, about 1 minute. Add the cream of tartar and salt, increase the speed to high, and beat until stiff peaks form, about 2 minutes more; set aside.
Place the corn syrup, sugar, water, and 3/4 teaspoon of the vanilla in a small, heavy-bottomed saucepan over high heat and stir with a rubber spatula until the sugar dissolves. Bring to a boil, swirling the pan occasionally (do not use the spatula to stir), until the mixture is light amber in color and registers 240°F on a candy thermometer. Immediately remove the saucepan from the heat.
With the mixer on low speed, very slowly pour the hot syrup into the egg-white mixture and continue beating until the sauce is smooth, glossy, and opaque and flows like molten lava, about 2 minutes total. Beat in the remaining 3/4 teaspoon vanilla. Transfer the sauce to a container, cover, and refrigerate until cold before using.