Place a saucepan on the stovetop, fill it with about 2 inches of water and bring it to a boil.

In a heatproof bowl (large enough to sit on top of the saucepan without touching the water) combine the egg whites, granulated sugar and cream of tartar. (Alternatively, you use a double boiler.)

Increase the heat to medium-high and hand-whisk the egg mixture continuously. As it heats, it will become frothy and thinner.

Keep whisking until the sugar has fully dissolved into the egg-white mixture.

To check, dip a clean finger into the mixture and rub it between your fingers. It should feel smooth with no sugar granules. Be cautious, as the mixture will be hot. If you're using a candy thermometer, the temperature should read 160F.

Once the sugar has dissolved, add the vanilla extract and whisk on high until stiff peaks form.

Your marshmallow fluff is now ready to use. Once it has cooled to room temperature, transfer it to clean jars to store or use immediately.