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. Add the vanilla extract if using and whisk for a minute to combine.
Add the cornflour to each bowl, and carefully fold it 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 round nozzle. Pipe the mushroom capes onto one baking sheet, allowing about a centimeter or so between each one. You can vary the sizes to resemble real mushrooms. If needed, you can use the back of a spoon to flatten out any kinks from lifting the piping bag.
On the second baking sheet, pipe the stems by pulling upwards whilst piping. Use a sieve to dust both trays of meringues with some cocoa powder.
Bake in the oven for 60-75 minutes. Check they are ready by lifting one of the meringues, it should come clean from the tray. 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.
Melt the chocolate over a bain-marie (double boiler). Using a skewer or cocktail sitck, pierce a hole into the underneath of all the mushroom caps. Using a pastry brush, coat the underneath with melted chocolate and then carefully stick a mushroom stem into the hole. Repeat this with the remaining mushrooms.