Wipe your mixing bowl and whisk with vinegar or lemon juice to make sure there are no traces of fat left on them.
Separate the eggs into the clean mixing bowl. If any pieces of yolks get into the whites, discard them. Let the egg whites come to the room temperature, about 1 hour. We will use the egg yolks for lemon curd later.
If you don't have superfine sugar, you can pulse granulated sugar in a food processor or blender a few times until the texture is super fine.
Preheat your oven to 300°F (150°C). Line a baking sheet with parchment paper.
Add cream of tartar and a pinch of salt to the egg whites and using your stand mixer, beat on medium high for about 1 minute until soft peaks form.
Slowly add the sugar, one or two tablespoon at a time, while continuing to beat the egg whites. If there are sugar granules stuck to the side of the bowl, scrape it with a spatula to make sure there aren't any pieces of sugar undissolved. When all the sugar is added, continue beating on high speed for about 8-10 minutes, until glossy and white and stiff peaks form. It means when you hold the whisk up, the meringue keeps its shape. Rub some of the meringue between your fingers to check if the sugar is dissolved, if it feels gritty you should beat it more. Continue checking every couple of minutes.
Fold in the vanilla extract and lemon juice, followed by cornstarch.
Pipe the meringue into 2.5 - 3 inch nests. You can use a star shaped piping tip. You can also spoon the meringue directly onto the parchment paper and form into 3 inch nests.
Use an offset spatula or a small spoon to create a small indentation in the center of each nest.
Place the meringue in the preheated oven and immediately reduce the oven temperature to 220°F. Bake for 1 hour and 15 minutes. Then turn off the oven and let the pavlova cool down in the oven (door closed) for 30 minutes. They will be crisp on the outside and soft on the inside.
Take the pavlovas out of the oven and let them cool down.