First, add the egg white and cream of tarter to a stand mixer and begin mixing on a med-low setting. Then, add the sugar, corn syrup, and water to a small sauce pot and give it a quick stir to combine. Turn the burner to between medium-high and let the mixture heat up until most of the sugar is melted and the syrup begins to bubble. (Do NOT stir the mixture once the heat is on. Too much agitation by outside objects could make the syrup seize)
Next, turn the heat higher a bit at a time to bring the syrup to 240°F (115.5°C) Once reached, immediately move the pan from the burner.
Keep an eye on the egg white while the syrup heats. It should have increased in volume and become an opaque white. You should be able to visibly see individual bubbles in the whipped foam. Texturally, you are looking for what would be just before soft peaks. Now, switch on the stand mixer to the lowest setting and very slowly pour the HOT syrup into the mixer.
When combined, you can begin turning up your mixer to a higher speed. Let your mixer run until the bubbles disappear into a glossy and thick, yet slightly soupy, viscosity. As the mixer incorporates air into the marshmallow fluff it will increase in volume even more.
Around the time that the fluff stays mostly inside the whisk attachment when you lift the stand mixer head, and only slightly ribbons down, add the vanilla extract and the salt. Mix until just combined and then begin roasting the fluff with a butane torch. To do this, I simply lift the stand mixer’s head up, torch the fluff caught there, lower the whisk, turn on the mixer for 5-10 seconds at medium speed, and then repeat the process until you are happy with the fluff’s flavor (see FAQ on flavor) and it has whipped to stiff peaks. The marshmallow fluff will go from white to a lightly beige/gray color and will have flecks of char studding throughout.