Line a baking tray with baking paper. Using a 20x 30cm tin or a ruler, trace out a roughly 20 x 30cm rectangle on your baking paper with a pencil - this gives you a guide for shaping the pavlova.
Preheat the oven to 180°C (not fan bake).
Wipe out your stand mixing bowl with a bit of lemon juice or vinegar to remove any bits of grease from last time you used it - you want your bowl and beater attachment to be super clean and dry.
Add the egg whites and salt to the bowl and beat using the whisk attachment on high speed until frothy.
Still on high speed, start adding the caster sugar, 1 tablespoon at a time, until it is all absorbed and the egg whites are stiff and glossy. This should take about 10 minutes, and when you rub a bit of meringue between your fingers you shouldn’t be able to feel any sugar granules.
Whisk in the vinegar. Sift the corn flour over the meringue and gently fold through with a spatula.
Use a big spoon to drop dollops of meringue onto your traced rectangle. Use an offset spatular similar to spread it gently into a rectangle, creating a few swirls and peaks as you go.
Place it in the oven. Immediately turn the heat down to 130°C and bake for 1 1/2 hours. Turn off the oven and leave it in the oven without opening the door until completely cool, ideally overnight.