Combine the water, sugar, and corn syrup in a medium heavy bottom saucepan fitted with a candy thermometer. Place over medium-high heat.
Do not stir, the mixture will bubble.
Place the egg whites and cream of tartar in the bowl of a stand mixer fitted with the whisk attachment or a large mixing bowl using an electric hand mixer with whisk attachment. Whip the egg whites to soft peaks, approximately 3-4 minutes.
When the sugar syrup reaches 240·°F, remove the heat and very slowly and in a steady stream, pout the sugar mixture into the egg whites while on medium speed. If you are using a hand mixer, you might need a set of extra hands.
When it is fully combined, continue to whisk, increasing the speed to medium-high. The mixture will first deflate, but will thicken back up. Continue to whip for 7-8 minutes, then whisk in the vanilla.
Allow the mixture to cool completely before using. Store in an airtight container at room temperature for up to 2 weeks.
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Notes
Make sure the bowl and whisks are grease and liquid free. You can even wipe them down with pain white vinegar, lemon juice or wiping alcohol if you allow enough time for the liquid to dry.