Place a saucepan on the stovetop, fill it with about 5 cm (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.

Position the bowl on top of the saucepan with boiling water. (Alternatively, you can opt for 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 71C/160F.

Once the sugar has dissolved, carefully pour the hot syrup into the grease-free bowl of a stand mixer. If you don't have a stand mixer, no worries - an electric hand mixer will do the trick! Transfer the hot mixture to a large, grease-free bowl before whisking. Just be prepared for it to take a minute or two longer to reach stiff peaks.

Add the vanilla extract.

Using the whisk attachment, start whisking the warm mixture at a high speed.

Keep whisking until the mixture thickens, turns pure white and forms stiff peaks. Depending on your method and equipment speed, it should take about 5-6 minutes of whisking to achieve stiff, glossy peaks. Keep an eye on the texture and stop whisking once the fluff reaches this stage.

Your marshmallow fluff is now ready to use. Once it has cooled to room temperature, transfer it to clean jars or use it directly for piping or spreading.