In a standalone mixer, whisk the eggs and cream of tartar together until soft peaks form. You can let this whisk while you boil the other ingredients.
In a medium heavy saucepan combine the water, corn syrup and sugar.
In a medium heavy saucepan, combine the water, corn syrup, and sugar. Stir constantly over medium heat until the sugar dissolves and it reaches a boil. Once boiling, stop stirring. Let the mixture continue to boil undisturbed for about 5–7 minutes. If you are using a candy thermometer, you are looking for the Soft-ball stage (240°F). The syrup must reach this exact temperature to properly thicken the egg whites and create those iconic glossy peaks.
Once the egg whites create thick peaks, carefully pour the syrup in a thin, steady stream into the whites with the mixer is on medium-low speed. Let it mix fully in between pours.
Add in the vanilla extract or any other spices and extracts you want to flavor the marshmallow fluff with.
When it’s all in, beat on high with the whisk attachment until thick. It will thicken up nicely!
Notes
Make sure the sugar mixture boils for a good 5-7 minutes. More if needed.Use a candy thermometer to give you accurate results. You'll boil the sugar on a soft-ball stage on a thermometer. Stir constantly so it doesn't burn.Add in spices or extracts to create your own flavors.Be very careful when working with hot sugar. Pour it slowly into the whipped egg whites and take your time doing it. Don't over whip the fluff or it will get too stiff to spread on desserts.If you have leftover marshmallow fluff you can freeze it to use later.Room temperature ingredients are always recommended, but not required.How to Toast Marshmallow FluffThe best way to get golden brown peaks is with a kitchen torch. It gives you that perfect toasted marshmallow look and flavor.If you don’t have a torch, place the fluff under the broiler for a quick toast. Watch closely so it doesn’t burn. Toasted fluff looks beautiful on cupcakes, cakes, or even sweet potato casserole.