Prepare Ingredients:
Separate egg whites into a clean, dry mixing bowl. Set aside.
Cook Sugar Syrup:
In a small saucepan, combine sugar, corn syrup, and water. Cook over medium heat, stirring until sugar dissolves. Bring to a boil without stirring until the syrup reaches 240°F (soft-ball stage) on a candy thermometer.
Whip Egg Whites:
While the syrup is heating, beat egg whites with cream of tartar and salt on medium-high speed until soft peaks form.
Combine Syrup and Egg Whites:
With the mixer running on low speed, slowly pour hot sugar syrup into the whipped egg whites in a thin, steady stream.
Beat to Stiff Peaks:
Increase mixer speed to high and beat until mixture is thick, glossy, and has stiff peaks. Beat in vanilla extract.
Cool and Store:
Let marshmallow cream cool to room temperature before using.