Place water, corn syrup and sugar in a medium saucepan. Stir to combine them.
Fit a candy thermometer to the pot and put it on medium-high heat. Don’t stir at this point or crystals will form
Wipe down your stand-mixer bowl, whisk and other equipment that you will need from now with lemon juice or vinegar to ensure that they are completely free of grease. [N.B. Even if there is a tiny speck of grease or egg yolk in the utensils, the meringue doesn’t whip]
Add the egg whites and cream of tartar to the bowl of the stand mixer
When the sugar syrup reaches 225°F, start whipping the egg whites mixture so that they form soft peaks. It will take around 3-4 minutes
When the egg whites are ready and the temperature on the candy thermometer reads 240°F, turn the mixer to medium and slowly pour the sugar syrup into the egg mixture in a thin, steady stream down the sides of the bowl. [N.B. You should do this very carefully as the stream should be perfect as it flows down the sides of the bowl without getting splashed by the whisk.]
Now set the mixer to medium-high and continue beating. The whites will deflate at first but will thicken and fluff afterward
Continue whipping for 7-8 minutes until the mixture becomes thick and fluffy
Add the vanilla and whip till the mixture fully cools down
You can now use it right away as filling or frosting or transfer it to an airtight container and store it for up to 2 weeks at room temperature. When stored thus, the spread needs to be whipped again to fluff it up before serving.