The first step is reducing the aquafaba, this can be done the night before or at least two hours in advance of making the recipe. Add the aquafaba to a saucepan and bring to a boil. Reduce the heat and allow it to simmer until it has reduced by half in volume. After 5 minutes of simmering, check the volume by pouring the liquid into a bowl on your weighing scales or measure the volume by cups (be careful as it’s going to be hot!). If there's more than half of the original liquid volume continue to simmer until it reduces to half.
Allow the aquafaba to cool to room temperature and pour it into a jar and refrigerate to chill.
Preheat your oven to 100°C (212°F). Line a baking pan with some parchment paper and dust with a thin layer of cornflour - this will make it easier to lift the meringues once they are ready and prevent them from sticking to the paper.
Take a large clean bowl, and make sure it's completely dry and free of any grease. Add the aquafaba and cream of tartar to a large bowl and whisk on high speed for 10 minutes until soft peaks form.
Add the caster sugar one tablespoon at a time while continuing to whisk and incorporate the sugar into the aquafaba. Whisk in each tablespoon for about 30 seconds before adding the next. Continue this process until you have incorporated all of the sugar, then whisk for another 10 minutes until thick and glossy.
Carefully fold the cornstarch through the meringue mixture using a spatula until it is just incorporated - do not over-mix.
Transfer the meringue to a piping bag fitted with a star nozzle and pipe nests onto the baking tray, leaving a well in the center. Allow a couple of inches between each next.
Bake in the oven for 2 hours. Once ready, leave the meringues in the oven for another 15 minutes with the door ajar. Take the tray out of the oven and allow the meringues to cool completely for another 30-60 minutes before removing them from the tray.