To the bowl of a stand mixture fitted with the whisk attachment (or using a handheld mixer), add the egg whites and cream of tartar. With the mixer on low speed, whisk until the eggs begin to foam. Slowly increase the speed to medium-high and mix until the eggs are doubled and have turned white with soft peaks. When you lift the whisk out of the mixture, the top of the peak should slightly droop (like soft-serve ice cream). This whole process takes about 3-4 minutes.
Once the syrup mixture reaches 240°F, remove it from heat.
With the stand mixer on low speed, slowly pour a thin stream of syrup into the egg whites. The egg whites will deflate at first, but will start to thicken and fluff back up as the syrup is added.
Once the mixture is combined, gradually increase to medium speed and continue to whisk for 7-8 minutes, or until the mixture is fluffy and shiny.
Once the mixture is fluffy and shiny, add vanilla and mix for an additional minute.
Serve immediately as a topping for your favorite desserts. You can also use this mixture in any recipe that calls for marshmallow fluff or marshmallow creme.
