Place water, sugar, and corn syrup in a medium saucepan. Stir to combine.
Insert a candy thermometer into the pot and heat over medium-high. Do not stir from this point on as crystals will form.
Wipe down your mixer bowl, whisk, etc with lemon juice or vinegar to ensure they are completely grease free.*
Place egg whites and cream of tartar in the bowl of a stand mixer.
When the sugar syrup reaches about 225°F, start whipping the egg whites to soft peaks. Approx 3-4mins.
When the whites are ready, the sugar syrup should be at 240°F. Remove from the heat, turn mixer to medium and very slowly and carefully pour the sugar syrup into the whites in a thin, steady stream.**
Once all of the syrup is in, set mixer to medium/high and continue whipping. The whites will deflate at first, but they will thicken and fluff up.
Continue to whip for 7-8 minutes, or until the mixture is thick and fluffy.
Add in vanilla and whip until the fluff has cooled completely.
Use right away as a frosting or filling or transfer into an airtight container and store for up to 2 weeks at room temperature.***
* If there is even a speck of grease (or egg yolk) the meringue will not whip. I wipe everything down with lemon juice before adding the egg whites. ** The sweet spot for this is getting the stream right between the whisk and the sides of the bowl, so the syrup doesn't get splashed by the whisk. *** The spread may need to be re-whipped to fluff it up again.