Preheat the oven to 350°F. Generously grease a 16x12-inch sheet pan, then line it with greased parchment paper.
In a stand mixer fitted with the whisk attachment, combine the egg whites, salt, and cream of tartar and beat on medium-high speed until medium peaks form, about 3 minutes.
Turn the mixer speed to medium-low and drizzle in the water. Slowly sprinkle in the sugar a little at a time; this may take a minute or so. Turn the speed to high and whip until very stiff, glossy peaks form, about 5 minutes.
Using a rubber spatula, gently fold the cornstarch, vanilla, and vinegar into the egg whites.
Evenly spread the pavlova batter in the prepared pan. Bake until the pavlova starts to turn a light caramel color, about 20 minutes. It will puff considerably while it bakes but will settle once it is out of the oven. Let the pavlova cool completely. (Just leave it in the pan, dust with confectioners’ sugar, and cover with a clean kitchen towel for up to 1 day.)