Preheat oven to 240 degrees F. Make sure the rack is placed in the middle of the oven. Trace a 9-10 inch circle on a piece of parchment paper (I used a cake pan as my stencil). Put the marked side face down on a baking sheet and set aside.
Start simmering about 1 inch of water in a saucepan over medium heat.
Whisk together the egg whites and sugar in a heat-safe bowl (I used the bowl of my stand mixer). Place the bowl over the simmering water, making sure the bottom of the bowl doesn't touch the water. Whisk gently but consistently to cook evenly. Keep whisking until the sugar is dissolved and the temperature is 160 degrees F (about 6 minutes).
Meanwhile, mix vinegar, cornstarch, and vanilla in a small bowl. Set aside.
Transfer the bowl with your egg mixture to the base of your stand mixer (careful- the bowl may be hot to touch). Use the whisk attachment and whip on high speed for about 4 minutes. The meringue is done with you reach stiff peaks and it is smooth and glossy. The color will be bright white.
Stop the mixer and scrape down the sides. Add your vinegar, cornstarch, and vanilla. Whip again on high speed until evenly combined, about 10 seconds.
Put a little dap of the sticky meringue on each corner of the parchment paper, facing down on the baking tray. This helps the paper stay in place when you are shaping your meringue.
Transfer the rest of the meringue into a pile at the center of the circle you drew. Use a spatula to spread out the meringue into the larger circle shape (it doesn't have to be perfect). You want the edges to be a little higher than the center (you want a little crater to fill with whipped cream). I did a little swirling at the edges to give it an effortless, rustic look.