Preheat the oven to 300 F (I did 280 F on convection).

You'll definitely want to use your stand mixer if possible, as this requires a massive amount of whipping. Take a paper towel with some white vinegar on it and wipe the inside of the mixer bowl (to make sure there aren't any traces of fat, which can affect the egg whites whipping up).

Add the egg whites and salt to the mixer and whisk on high until stiff peaks form.  

Once you have good stiff peaks, start adding the sugar, 1/4 of a cup at a time and whisk for about 1 minute after each addition.  You want the sugar to be dissolved before adding the next amount.  You can test this by gently rubbing a tiny bit of the mixture between your fingers---if it's grainy, keep whisking.
After the last of the sugar has been added, whisk on high for 3 minutes.  The mixture should be very thick and glossy at this point.

Add in the cornstarch, vanilla, and vinegar, and whisk until just combined.

Line a tray with baking parchment; you can put a tiny dab of the meringue underneath the corners to keep it from sliding (Mary Berry trick!).

Scoop out the meringue mixture, forming about 6 individual portions.  You can use your spoon or a spatula to gently shape each into a nice bowl-like shape (see pics).  You can add swirls to the outside, or even pipe some of the mixture for extra pizzazz.

Place the meringue in the oven on the middle shelf and reduce oven temperature to 250 F (230 F convection).

Cook for at least 1 hour, but it could be up to an hour and a half.  DO NOT OPEN THE OVEN UNTIL AT LEAST AN HOUR HAS PASSED!  Use the oven light to see the pavlova if necessary.  The pavlova should be lightly browned and dry to the touch when it's done (at that point you can quickly open the oven door and touch it).

When it's done, turn off the oven, and leave the pavlovas in the oven with the door closed for (ideally) 2-3 hours until the pavlovas is cooled down.

Either the night before or at least while the pavlova bakes, make the grapefruit curd.  See the link in the post for the grapefruit curd recipe, it's super easy and only takes 10 minutes!  Let it cool completely so it can solidify.

Serve the pavlova with the grapefruit curd, berries, and whipped cream.