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.