four 8-inch round cake pans; two 5-inch round cake pans; a piping bag fitted with a plan 1/2-inch tip; kitchen-dedicated tweezers; an offset spatula
For the cake: Preheat the oven to 350 degrees F. Grease four 8-inch round cake pans and two 5-inch round cake pans with flour-based baking spray
Sift the flour, baking powder and salt into a large bowl. Beat the butter in the bowl of an electric mixer on medium speed until smooth. With the mixer running, gradually add the granulated sugar and beat until the mixture turns pale and fluffy, about 3 minutes. Reduce the speed to low and add the flour mixture in 3 additions, alternating with the milk. Begin and end with the flour mixture. Stir in the vanilla. Beat until just combined. Do not over-mix.
Beat the egg whites in a clean mixer bowl on medium speed until stiff peaks form, about 5 minutes. Gently fold the egg whites into the batter in 3 additions.
Place 2 cups of cake batter in 4 separate bowls. Tint the first bowl with the green food color, the second bowl with 1 tablespoon of the purple food color and the third bowl with the orange food color. Combine the cocoa and water in a small bowl, stirring well to create a paste, then add it to the fourth bowl of batter. Fold in the black food color and stir together well until the batter is dark and no streaks of white batter remain. Pour one color of batter into each of the prepared 8-inch pans.
Tint the leftover batter with the remaining 1 tablespoon purple food color and place 1 cup of batter in each of the prepared 5-inch pans.
Bake until the cakes spring back when pressed in the center, about 25 minutes for the 5-inch cake layers and 30 to 35 minutes for the 8-inch cake layers. Run a knife around the edges of the cakes and the pans and turn them out on wire racks to cool.
For the frosting: In a stand mixer fitted with the whisk attachment, mix together the confectioners' sugar and butter. Begin on low speed until crumbly, then increase to high and beat for 3 minutes until smooth and fluffy. Add the vanilla extract and beat again for another minute. If you find the buttercream is too stiff, you may add milk or heavy cream 1 tablespoon at a time until the mixture is spreading consistency. Place about 2 cups of buttercream in a piping bag fitted with a plain 1/2-inch piping tip; set aside for later use. Tint the remaining frosting with the black gel food color.
Level the tops of the cakes with a serrated knife or a cake leveler. Cover each cake layer with a small amount of black frosting. Begin with the black chocolate cake layer on the bottom, followed by the purple, then the green and ending with the orange on top. Cover the entire cake with black frosting using an offset spatula. Cover a purple 5-inch cake layer with frosting and stack the second on top. Cover with frosting using an offset spatula. Refrigerate both cakes until the frosting is firm, about 30 minutes.
Stack the 5-inch cake tier on top of the 8-inch cake.
For the marshmallow web: Melt the marshmallows in the microwave for about 30 seconds or until they puff and lose their shape. Stir well until deflated. Let the marshmallow cool slightly. Dip fingers (or gloved hands for easy cleanup) into the marshmallow mixture and stretch until thin and web-like. Wrap the marshmallow around the cake. Repeat the process until the marshmallow web effect covers both tiers of the cake. Let stand until firm, about 10 minutes.
For the assembly: Cover a baking sheet with parchment paper. Use the reserved bag of frosting to pipe a 2- to 3-inch mound of frosting. This will be a ghost body. Top with a second 1-inch mound of frosting for the ghost head. Repeat the piping process, varying the sizes of ghosts for visual interest, until you have about 20 ghosts. Add 2 black sugar pearls to each ghost face using a pair of kitchen-dedicated tweezers. Use a single black confetti sprinkle to create mouths on some of the ghosts. Transfer the pan to the freezer and chill until firm, about 1 hour.
Use an offset spatula to transfer the ghosts to the edges of the cake.
Bring the cake to room temperature before serving. Use kitchen-dedicated scissors to snip the firm marshmallow webbing before cutting into slices.