Add water, sugar, and corn syrup to medium sauce pan and turn heat to medium-low. Stir until the sugar has dissolved.
Once sugar has dissolved, raise heat to medium and add candy thermometer. Do not stir any longer or else sugar crystals will form. Let candy boil until it thermometer reads 240 degrees. Remove from heat.
Add egg whites and cream of tartar to a clean bowl fitted for a stand mixer. Attach whisk attachment. Beat egg whites on medium until soft peaks form, about 3-4 minutes.
Turn mixer speed to low. Very carefully, pour heated sugar mixture in a thin, slow stream as the mixer beats it into the egg whites. Once all the sugar mixture is combined with the egg whites, turn up speed to medium high to fluff up the mixture again for about 6-8 minutes. The mixture is done with it's thick, fluffy, and glossy.
Store leftovers at room temperature for up to 2 weeks. The fluff will start to deflate over time. You can whip it back up if you desire.