The next day, in a large bowl, lightly beat the aquafaba until it starts to bubble. Add 4 cups of the powdered sugar and mix well. Test the consistency of the icing on a plate. It should be thick enough that it doesn’t slide down the plate easily. If it seems too thin, add 1 more cup of powdered sugar. The consistency should be considerably thicker than the icing used to decorate cookies.
If you are making complete skulls, leave some of the royal icing white, and use it to glue the front and back of the skull, after it has dried for the initial 24 hours. After you have glued it together, let dry a bit before decorating it.
If you live in a humid climate the sugar skulls will take longer to dry, and you will most likely have to add less aquafaba. The longer you let the skulls dry the better. The sugar skulls are not meant to be eaten. They are for decoration. Royal icing recipe adapted from The Blenderist.