Place water, sugar, and corn syrup in a medium saucepan, stir with a whisk until combined.
Insert a candy thermometer into the pot and heat over medium-high. Do not continue to stir
Place egg whites and cream of tartar in standing mixer.
When the sugar syrup reaches about 225 degrees, start whipping the egg whites into soft peaks
When the whites are soft peaks, this is when the sugar syrup should be at 240 degrees
Remove from heat, turn mixer to medium and very slowly pour the sugar syrup into the egg whites in a thin, steady stream.
Once all of the syrup is poured in, set mixer to medium/high and continue whipping.
Continue to whip for 7-8 minutes, or until the mixture is thick and glossy.
Add in vanilla and whip until the fluff has cooled, about 2 minutes.
Scoop some of the marshmallow fluff into the piping bag and place the rest into a Tupperware.