Ice cake one day in advance. Prepare buttercream icing following recipe directions. Tint portions black, orange and green.
For pumpkin, use 9 in. angled spatula and orange icing to assemble cake halves. Use knife to cut away a portion of front of pumpkin at bottom for mouth, 5 in. x 2 1/5 in. and trim leaves off top of cake.
Use tip 3 and black icing to outline eyes, eyebrows and pipe dot nostrils. Use 9 in. tapered spatula and black icing to ice mouth area. Use finger dipped in cornstarch to pat smooth. Use tip 21 and orange icing to cover cake with stars. Use tip 21 and green icing to pipe a pull-out rosette for stem.