Preheat oven to 300 degrees. Line a baking pan with parchment paper and draw a 9" round circle on the paper (I trace a 9" round cake pan).
Place egg whites and salt in a large bowl or base of stand mixer (see notes above about temp and separating of eggs).
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 3.5 minutes.
Add water, vanilla, sugar, cream of tarter, and corn starch. Beat on high speed until stiff peaks form (peaks that easily stand and hold their shape). This will take about 2 minutes.
Use spatula to fold in cream of tarter and corn starch.
Dump mixture into center of circle on parchment paper. Use spatula to spread out mixture to edges of circle. Then, use spatula to move mixture from center of mixture to the outside, leaving about a 5" round circle (make sure to leave about 1/2" thickness of mixture on the bottom).
Next, starting at outside of circle, move spatula from bottom to top in a vertical direction, to form peaks. Once done you should have peaks of the mixture around a 5" round central indentation.
Bake for 30 minutes. Then, without opening oven, reduce heat to 250 degrees and bake for another 70 minutes, or until golden in color (Note: Do not open oven at all during this time).
Once time is done, turn oven off. Again, do NOT open oven. Let cool completely in oven for about 4 hours, or overnight.
Remove from oven and then remove from parchment paper. Transfer to serving plate or cardboard round.