Start by reducing the aquafaba, add it to a saucepan and bring to the 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.
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 the caster sugar and 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 a 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.
Whisk the aquafaba mixture for another 5 minutes before transferring it to a piping bag fitted with a nozzle of choice.
Use the Italian meringue as a topping for brownies, baked Alaska and other desserts.
You can store this meringue in an airtight container in the fridge for up to 24 hours before use.