- Adjust the oven racks to upper-middle and lower-middle positions and heat the oven to 225°F Line 2 baking sheets with parchment paper.
- Combine cornstarch and sugar in a small bowl.
- In a stand mixer with the whisk attachment, beat the egg whites, vanilla, and salt at high speed until very soft peaks start to form and the foam just starts to gain volume (peaks should slowly lose shape when whisk is removed) about 30-45 seconds.
- Reduce the mixer speed to low and slowly add the sugar mixture in a steady stream down the side of the mixer bowl. This process should take about 30 seconds.
- Stop the mixer and scrape down the sides and bottom of the bowl with a rubber spatula.
- Turn the mixer back on to high speed and beat until glossy, stiff peaks have formed, about 30-45 seconds.
- Working quickly and carefully, put meringue mixture into a pastry bag fitted with 1/2" tip. Pipe the meringues into 1 1/4"-wide mounds about 1" high. Try to fit 6 rows of 4 meringues on each baking sheet.
- Bake for 30 minutes, then rotate pans front to back, top to bottom, and bake for 30 more minutes.
- Turn off the oven and allow the meringues to cool for at least 1 hour (or 2 if a humid or rainy day). Remove from oven and allow to cool to room temperature before serving (about 10 minutes).