Bring egg whites to room temperature. Place in the bowl of stand mixer.  Beat, increasing speed to medium-high and sprinkle in cream of tartar.  Continue beating until the eggs start to get light, airy, and frothy.  Keep mixer running and slowly pour in 2 ounces of the sugar.  Continue to beat until firm, but not stiff peaks form.

In a medium saucepan, combine water, sugar and corn syrup over medium heat.  Use a thermometer to cook the sugar until mixture gets to 240°F, maintaining a consistent boil. Once at 240°F, remove from the heat immediately.

Start the mixer on second speed and slowly pour the syrup in until all of the syrup has been added. Stop the mixer and scrape down the bowl if needed, and then increase speed to medium-high and beat for another 5 minutes. The mixture should have expanded and you should now have a white creme that's able to hold some shape. 

Add the vanilla, wipe down the bowl and beater, and beat for about 1 more minute, until the mixture is even in color again and has reached the desired consistency of marshmallow creme.

Store fluff in an airtight container in the refrigerator