In a medium-sized saucepan combine ⅓ cup water, ¾ cup sugar and ¾ cup corn syrup. Insert a candy thermometer into the mixture and heat over a medium-high flame. Do not stir the sugar water as it cooks, and cook until the mixture reaches 240°F. While waiting, continue onto the next step.
While the sugar syrup is cooking, place the egg whites and cream of tartar in the bowl of your stand mixer. Beat the whites to soft peaks. This typically takes 3-4 minutes.
When the sugar syrup reaches 240°F, remove immediately from the heat. With the mixer running on medium speed, carefully pour the syrup in a thin stream into the egg whites, whipping the whole time.
Continue to whip the mixture for 7-8 minutes or until very thick and glossy. The heat of the sugar mixture will deflate the whites initially, but they will eventually thicken and fluff up.
Add 1 tsp vanilla and whip until incorporated.
Transfer the fluff into an airtight container like a mason jar. Store for up to 2 weeks at room temperature. The spread can be re-whipped by hand if it starts to separate.