In advance: Make hair. Tint fondant black. Knead in 1/8 teaspoon of Gum-Tex. Roll out 1/16 in. thick. With knife, cut 1/4 in. wide strips in various sizes from 2 in. to 6 in. long. Wrap strips around cookie treat sticks. Let set for one minute. Slide off stick. Let dry 24 hours on cornstarch-dusted board, leaving some straight and curving others to the left or right.
Bake and cool skull cake in 2-piece pan. Attach cake halves with icing.
Using tip 3 and black icing, outline and pipe in mouth and eyes. Pat smooth with finger dipped in cornstarch.
Using tip 10 and white icing, pipe ball eyes. Using tip 6 and black icing, pipe dot pupils.
Using tip 16 and green icing, cover skull with stars.
Insert hair in top of cake. Pipe tip 16 stars in back for support if needed.
The star tip creates the most celebrated, easily accomplished decorations! The serrated edges of the star tip make ridges in the icing as you squeeze it out. Because fill in requires so much piping from the same bag, keep replenishing icing in bag.