1Place a saucepan on the stovetop, fill it with about 2 inches of water, and bring it to a boil.
2In a heatproof bowl, combine the egg whites, granulated sugar, and cream of tartar.
3Place the bowl on top of the saucepan, ensuring it does not touch the water, and hand-whisk the egg mixture continuously over medium-high heat until the sugar has fully dissolved.
4Once the sugar has dissolved, add the vanilla extract.
5Transfer the mixture to a stand mixer with a whisk attachment.
6Whisk on high speed until stiff peaks form.
7Once cooled to room temperature, transfer the marshmallow fluff to clean jars for storage in the fridge or use immediately.