In a large bowl, lightly beat the aquafaba until it starts to bubble.
Pour in the sugar, and use your hand to mix well and incorporate the sugar and the aquafaba. It should have the consistency of wet sand, almost like you are going to build a sand castle.
Make sure your mold is clean and dry. Press the sugar mix into the mold. Use a spoon to scoop out some of the sugar from the back of the skull. This will make the skull less heavy.
Press the cardboard square against the mold and flip the mold, to have the skull facing you. Lift the mold, and carefully place the cardboard with the skull on it on a sheet tray.
Repeat this process with the rest of the sugar. If you want to make a complete sugar skull use both the skull molds.
Leave to dry for at least 24 hours.
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.
Separate the icing into 4 small bowls. Add your food coloring of choice and mix well.
Pour each bowl of icing into a disposable piping bag. Secure with a rubber band, and cut a tiny bit off of the tip of the bag. Test the amount of icing that comes out before decorating your skull.
Decorate your skull however you desire. I like to use sequins for the eyes. Let dry 24 hours.
Making a complete skull
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.