Preheat oven to 250 degrees. Line a large baking sheet with parchment paper and draw two 8" round circles (make sure circles are at least 4" apart, if your baking pan is not large enough for this, you'll need to use two baking pans).
Place egg whites and salt in a large bowl or base of a stand mixer.
Use whisk attachment to beat on high speed until medium-stiff peaks form (stiff enough that it forms peaks, but peaks that don’t stand easily on their own). This will take about 3-4 minutes.
Add water, vanilla, and sugar. Beat on high speed until stiff peaks form (peaks that easily stand and hold their shape). This will take about 3 minutes.
Use spatula to fold in cream of tartar and cornstarch.
Divide mixture evenly between the two circles. Use spatula to spread out mixture to edges of the circles.
Bake for 1 hour. Do not open oven during cooking time.
Once time is done, turn oven off. Again, do not open oven. Let cool completely in oven for 3 hours, or overnight.
Remove from oven and then gentle peel parchment paper from bottom of the meringue.