Start by reducing the aquafaba, add it to a saucepan and bring it to a boil. Reduce the heat and allow it to simmer until it has reduced to half a cup in volume. After 5 minutes of simmering, check the volume by pouring the liquid into a measuring cup. If there's more than half a cup of liquid continue to simmer until it reduces to the required half cup. Pour the reduced aquafaba into a jar and refrigerate. This can be done the night before or at least one hour in advance of making the recipe.
Line a 9-inch pan or similar with parchment paper, creating a sling to make it easy to lift out the marshmallows once set. Very lightly grease the pan with a neutral oil. To a small bowl, add the powdered sugar and cornstarch and whisk to combine.
Using a fine-mesh sieve, dust a layer of the sugar-cornstarch mixture on the base and sides of the lined pan. Set aside and reserve the remaining sugar-cornstarch mixture for later.
Make the marshmallows
Take a large clean bowl, making sure it's completely dry and free of 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 200g sugar and 60ml water to a saucepan, gently stir and allow the mixture to sit for a minute until the sugar dissolves into a paste.
Place the saucepan on medium-high heat and allow the mixture to come to a simmer and bubble. DO NOT STIR throughout the entire process. Place your candy thermometer into the syrup and continue to simmer until the mixture reaches 116°C (240°F) - about 10-12 minutes.
Once the sugar syrup has reached the correct temperature begin to add it to the whipped aquafaba. Slowly drizzle in a little at a time and continue to whisk until the mixture is incorporated. Add the vanilla extract and whisk for another 5 minutes.
To a saucepan, add the remaining 200ml water, 50 g sugar, and agar-agar powder and whisk well to combine. Place over high heat and bring to a simmer, cook for 3 minutes while whisking contiuously.
Slowly drizzle the agar mixture into the whipped aquafaba, a little at a time, and continue to whisk until the mixture is incorporated.
Transfer the marshmallow mixture to the lined and dusted cake pan. Cover loosely with a clean tea towel or paper towel and allow it to set at room temperature for at least 6 hours (overnight works well).
Once set, dust the marshmallow with some more of the sugar-cornstarch mixture. Carefully lift the marshmallow out of the pan and cut it into servings. You can roll the individual pieces in any leftover sugar-cornstarch mixture if desired.
Storage and serving
Store at room temperature in an airtight container lightly dusted with more sugar-cornstarch mixture. They will keep for between 7-10 days.
Notes
Aquafaba: Drain 2x 400ml cans of unsalted chickpeas, use only the liquid part.
Some white sugar in the US is not vegan, if in doubt use organic sugar.
Agar-agar powder can be substituted with vegan gelatin in the form of carrageenan like vege gel or veggieset.