Preheat the oven to 300°F (We will reduce the temperature once we put the pavlova inside). For sensitive recipes such as pavlova, It's best to have a oven thermometer to make sure the temperature is accurate. Line a baking sheet with parchment paper, draw a 7 inch (18 cm) circle on the back side of the parchment paper, I used a plate to draw the circle. We will use this circle as our guide to spread the meringue into a perfect circle.
Wipe your mixing bowl and whisk with vinegar or lemon juice to make sure there are no traces of fat left on them.
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.
Separate the eggs into a 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.
Add four room temperature egg whites and cream of tartar into a mixing bowl (I use a kitchenAid mixer), and start mixing on medium low (4 on a 10 speed mixer) until it reaches medium peaks. It takes about 5 minutes.
Once the egg whites are at the medium peaks stage, start adding the sugar 1 tablespoon at a time and beat on high speed. After adding each tablespoon, wait about 30 seconds so that the sugar is dissolved and incorporated in 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 until it's 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 it every couple of minutes.
When the meringue reached stiff peaks and all the sugar is dissolved, add vanilla extract and beat for a few seconds until incorporated.
Gently fold in lemon juice and cornstarch.
Transfer the meringue onto the prepared baking sheet but don't use the meringue on the top side of your mixing bowl where there may be some undissolved pieces of sugar left. Spread it with a spatula, using the circle you drew as a guide. You can also use your spatula to create vertical lines on the sides.
Place the meringue in the preheated oven and immediately reduce the oven temperature to 250°F. Bake for 75 minutes. Then turn off the oven and let the pavlova cool down in the oven for 2 hours or overnight.