Place the chocolate chips in a large bowl and melt over a double boiler until all the chocolate has melted. You can also melt it in 30 sec. intervals in the microwave, making sure to stir between each interval.
Pour popped amaranth into the bowl with the chocolate and mix well. with a wooden spoon, to incorporate.
Wet the mold a little bit with a moistened paper towel and press the amaranth chocolate mix into the mold.
Repeat this process with the rest of the mix. Let chocolate set for 30 min.
While de chocolate is setting, 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.
Decorate your skull however you desire. The icing will take about 30 min. to set.