Preheat the oven to 275F. Line a large baking sheet with parchment paper.
Add the egg whites and lemon juice to a mixing bowl. With a hand mixer or standing mixer with whisk attachment, beat on high speed until soft peaks are formed (about 2-5 minutes, depending on speed.)
Adjust the speed to medium high, then slowly add the sugar, slowly sprinkling 1 tablespoon at a time until you run out. After this, beat the mixture for approximately 5 minutes on medium-high speed until stiff peaks are formed. You want that shiny marshmallow fluff look and for the "batter" to hold it's shape. It should not feel liquidy. If it feels liquidy, keep whipping. If you keep whipping it and it STILL feels liquidy, add another tablespoon of sugar. Repeat until stiff peaks form.
Adjust the speed to medium. Add the cornstarch and vanilla extract. Mix until everything is combined, about 1 minute.
Using a spoon, gently scoop about 2-3 spoonfuls of the meringue mixture onto the lined baking sheet. Use the back of your spoon to lightly spread it into a circle, creating a deeper centre and taller border. Repeat until you are out of meringue mixture.
Bake in the oven for 30 minutes. DO NOT OPEN THE OVEN when it is done! Turn the heat off completely. Leave them in the oven for another 30 minutes. Even though the heat is off, they are still baking, so it's important to keep the oven closed. Once the 30 minutes is up, open the oven and check to make sure they are hardened. They should be pale and hard on the outside, but soft and chewy on the inside.
Allow to come to room temperature for 30 minutes - 1 hour before serving. They are best eaten the same day, but you can store on a plate or cake stand, lightly covered at room temperature for up to 1-2 days.