One day in advance: Make cookies. Prepare cookie dough and roll out 1/8 in. thick. Cut one 3 in. pumpkin cookie and two 2 1/4 in. cookies. Bake and cool cookies.
Prepare royal icing and tint portions green, orange and black. Use tip 2 and full-strength green royal icing to outline stems, and tip 2 and full-strength orange royal icing to outline pumpkins. Use tip 2 and thinned green royal icing to fill in stems, and tip 2 and thinned orange royal icing to fill in pumpkins. Let dry on waxed paper overnight.
When dry, use tip 3 and full-strength black royal icing to pipe facial features. Let dry. Attach cookie sticks to back of cookies with tip 2 and full-strength orange royal icing.
Individually prepare three boxes white cake mix following package directions; divide batter into two portions. In one portion of batter, gently stir in Leaf Green Icing Color to desired shade. (Use clean toothpicks to transfer color from jar to batter.) Repeat with Orange Icing Color to color second portion.
For bottom layer, place clean divider in the first prepared pan. Pour green-colored batter in outer and center sections and orange-colored batter in the middle section. Fill sections 2/3 full. Remove ring from pan by carefully lifting straight up on handles. Rinse and completely dry ring.
For top layer, repeat step 4 in second prepared pan. Remove ring from pan by carefully lifting straight up on handles. Rinse and completely dry ring.
For middle layer, place clean divider in the third prepared pan. Pour orange-colored batter in outer and center sections and green-colored batter in middle section. Fill sections 2/3 full. Remove ring from pan by carefully lifting straight up on handles. DO NOT PUT RING IN OVEN.
Bake and cool cake following package directions. Prepare buttercream and chocolate buttercream icings. Tint portion of chocolate icing black; tint portion of white icing orange and green. Leave a portion white. Stack and position cake on foil-wrapped cake circle. Ice cake smooth with chocolate buttercream icing and spatula.
Divide cake into eighths using Cake Dividing Chart and toothpick. At each division mark, use tip 8 and white buttercream icing to pipe ghosts? bodies and tip 2A to pipe ghosts? heads. Use tip 2 and black buttercream icing to pipe facial features.
Use tip 32 and orange buttercream icing to randomly pipe pumpkins on bottom border between ghosts. Use tip 3 and green buttercream icing for stems. Use tip 16 and green buttercream icing to pipe pull-out star grass.
Sprinkle confetti on cake top. Insert pumpkin cookies in cake center.