Put the egg whites and sugar in the bowl you'll use to whip them up later.
Place the bowl in a bain marie on medium heat without the water touching the base of the bowl.
Use a whisk to move the egg whites every couple of minutes, trying also to scrape the sides of the bowl.
Remove it from the heat when you put your finger inside and don't feel any granules of sugar left. Be particularly careful to check the bottom of the bowl. Alternatively, if you have a food thermometer, check that the temperature has reached 79°C/174°F, this is the most precise way to do it.
Beat on high speed until the egg whites come to room temperature.
Add the lime juice and mix. Then add the cornstarch and cocoa powder and mix into the egg whites.
Draw a circle at the back of baking paper using a pencil that is a bit smaller to the plate or cake stand you'll be using later to present it in. This will help you to stay within that diameter.
Stick the tips of the paper to the baking tray using a bit of the meringue.
Put the meringue at the centre of the circle you drew and spread it around using a spoon, forming a center at the centre as you go. Check out the video for further reference.
You can leave it like this for a rustic look or go around with a spatula to flatten it and then use the tip to add some texture.
Take to a preheated oven at 100°C/210°F for 1.5 hours.
Leave to cool down inside the oven to prevent cracking (some cracking is completely normal). This will take 4-6 hours.