Spray one 8-inch-round cake pan and two 6-cup Bundt pans with cooking spray. Beat the cake mixes, eggs, buttermilk and oil with a mixer; divide among the pans. Bake until a toothpick comes out clean, about 25 minutes for the round cake and 30 to 35 minutes for the Bundts; let cool.
Tint the vanilla frosting with orange food coloring. Trim the flat sides of the Bundt cakes and the top of the round cake to make level. Turn 1 Bundt cake upside down; spread with frosting. Top with the round cake, more frosting and the other Bundt cake, then frost the whole cake and chill 30 minutes.
Lightly dust a sheet of wax paper with cornstarch. Roll out the fondant on the paper into a 21-by-8-inch rectangle, about 1/8 inch thick. Cut into 12 strips, 8 inches long and 1 3/4 inches wide, tapering the ends as shown.
Attach the fondant strips to the cake, lightly pressing and smoothing them from the base of the cake upward. Refrigerate the cake 30 minutes.
Draw jack-o'-lantern eyes, a nose and a mouth on paper and cut them out; attach to the cake with toothpicks. Score around the cutouts with a paring knife, then remove the toothpicks and paper.
Trace the scored lines with the knife, cutting through the fondant and slightly into the cake. Pry out the pieces with the tip of the knife.
Cut a 1-inch-deep circle out of the top of the cake. Put the dark chocolate frosting in a resealable plastic bag; snip the corner and pipe the frosting into the cutouts.
Twist the 2 strings of licorice together and insert into the top of the cake to make a handle. Fill the top of the cake with snack-size candy.