Prep the dry ingredients: Sift the flour, cornstarch, and baking powder into a medium bowl. Set aside.
Whip the aquafaba: Pour the aquafaba into the bowl of a stand mixer fitted with a whisk attachment. Whip on high speed until stiff peaks form. Slowly add the caster sugar a little at a time, continuing to whisk until thick and glossy (meringue-like).
Add the flavor: Gently mix in the vanilla and melted coconut oil. If using lemon zest, fold it through.
Fold in the dry mix: Add the sifted dry ingredients and fold gently with a spatula until smooth and just combined. Avoid overmixing.
Pipe: Transfer the batter to a piping bag fitted with a round nozzle. Line a baking sheet with parchment paper. Pipe even lines onto the sheet, leaving space between each. Aim for 20-24 in total.
Dust: Dust the tops with icing sugar.
Bake: Bake at 150°C for about 20 minutes, or until the edges turn lightly golden.
Cool: Leave on the tray for 5 minutes, then transfer to a wire rack to cool completely. Store in an airtight container once fully cooled.