Preheat your oven to 180°C (356°F) and line 2 cookie sheets or baking trays with a sheet of parchment paper. You can very lightly grease the baking sheet with oil as this will make it super easy to remove the baked cookies, oil spray works best.
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 or stand mixer and whisk on high speed for 5 minutes until soft peaks form.
Slowly add in 3.55oz (100g) of caster sugar, one dash at a time while continuing to whisk and incorporate the sugar into the aquafaba. Continue whisking for another 3 minutes until it reaches stiff peaks.
In a separate bowl, whisk together the remaining 2.45oz (70g) sugar, coconut milk, sunflower oil, and vanilla extract until combined. Use a spatula to carefully fold the whipped aquafaba into the bowl the coconut milk mixture, in 3-4 increments. Be careful not to over-mix.
Mix together the flour, baking powder, and salt and carefully fold them into the aquafaba mixture. You can do this in 2-3 increments, again be careful not to over-mix.
Transfer the mixture to a piping bag fitted with a small round ½" nozzle and carefully pipe cookies in strips about 3" long. Leave at least 2 inches between each cookie as they will expand once baked.
Bake for 12-15 minutes, until the cookies begin to brown on the edges. Allow them to cool on the tray for about 5 minutes before transferring them to a cooling rack to cool completely. Repeat the process with the remaining batter *the full recipe will make about 4 cookie sheets in total). Once cooled, you can coat them in some more caster sugar if desired.