Preheat oven to 350ºF.
With an electric mixer or stand mixer, whisk egg whites with salt until the whisk leaves "trails" in the mixture.
Slowly add in sugar, one tablespoon at a time, while beating for at least 10-15 seconds after each addition until all the sugar is dissolved.
Stop beating when the meringue reaches stiff peaks. Rub a bit of meringue in between your thumb and index finger - it should feel smooth without being gritty.
Sprinkle cornstarch, vanilla, and vinegar onto meringue and fold with a rubber spatula until combined.
Transfer meringue into a piping bag fitted with a closed star tip.
On a baking sheet lined with parchment paper, pipe a filled-in circle for the base of the pavlova. Pipe a ring on top of the outer edge of the circle to create the "walls" of the pavlova and an empty space in the middle for the filling.
Put the baking sheet in the oven and immediately turn oven down to 300ºF. Bake pavlovas for 30 minutes.
Turn off the oven and leave them in the oven for another 30 minutes until they peel cleanly off the parchment paper.
Set aside to cool completely. Fill with blood orange curd just before serving.