Bake and cool cakes. Place cakes on cooling grid positioned over waxed paper-covered cookie sheet.
Tint one can icing copper. Heat in microwave at defrost setting (30% power) for 20-30 seconds; stir. Repeat until consistency of icing will pour. Pour over cakes. Let set.
Tint portions of icing black and green; leave some white. Use tip 3 and black icing to outline and fill in mouth. Pat smooth with finger dipped in cornstarch. Use tip 3 and white icing to pipe in teeth. Pat smooth with finger dipped in cornstarch.
Use tip 4 and white icing to pipe ball eyeballs. Use tip 3 and black icing to pipe in pupils. Use tip 2 and black icing to pipe outline eyebrows. Use tip 2 and copper icing to pipe dot nose.
Use tip 4 and green icing to pipe ball spider body. Use tip 2 and white icing to pipe dot spider eyeballs. Use tip 1 and black icing to pipe dot pupils and outline eyebrows. Use tip 1 and green icing to pipe outline legs.