In an 8 by 8 pan, dust with icing sugar and cornstarch. Set aside.
In a mixing bowl, add the aquafaba, cream of tartar and vanilla extract. Using a hand mixer or stand mixer, combine until stiff peaks are formed, approximately 10 minutes. Set aside in a cool place while you prepare the agar agar syrup mix.
In a saucepan, add the water and agar agar and bring to a boil, stirring constantly. Add the sugar and maple syrup, and lower the heat to medium. Allow to cook for another couple of minutes until everything is combined and warmed.
Begin stirring the aquafaba meringue again with the hand mixer, and slowly pour in the sugar, agar agar “syrup”. Make sure it is well incorporated. The mixture will be slightly shiny.
Pour the marshmallow mix into the dusted baking pan, and set in a cool place for 2 hours. Slice into marshmallow shapes and dust with more icing sugar as desired. Store in an air tight container on the counter for up to 2 weeks.