Add the egg whites, sugar, corn syrup, cream of tartar, and salt in the clean bowl of an electric stand mixer and nestle it in a saucepan of simmering water over medium-high heat. Do not allow the bottom of the bowl to touch the water. Whisk until the mixture is opaque, frothy, the sugar is melted (rub some between two fingers, if it feels gritty, keep whisking) and warm to the touch. Remove the bowl from the heat and transfer it to the stand mixer fitted with the whisk attachment. Mix on medium-high speed until stiff and glossy and cool to the touch, about 3-5 minutes. Add in the vanilla and mix for 20 seconds more. Welcome to your new favorite marshmallow meringue heaven.