four 7- or 8-inch round cake pans; a cake leveler, optional; 4 piping bags; 2 large closed star decorator tips; a cake comb with 1/2-inch teeth; a small kitchen-dedicated spray bottle; a kitchen-dedicated art brush; 2 cake boards
For the vanilla-buttermilk cake layers: Preheat the oven to 350 degrees F. Coat four 7- or 8-inch round cake pans with flour-based baking spray. Set aside.
Whisk together the cake mixes, flour, granulated sugar and salt in a large mixing bowl. In a 4-cup measure, stir together the buttermilk, water, eggs and extract. With a hand mixer running on low speed, gradually pour the liquid ingredients into the dry ingredients. Scrape the bowl down and mix again. Divide the batter evenly between two bowls. Tint one bowl with the green food color and the other with the purple food color. Mix well until no streaks of white cake batter remain.
Divide the green cake batter between two of the prepared pans, then divide the purple batter between the other two prepared pans. Fill each pan no more than three-quarters full. Bake for 40 to 45 minutes if using the 7-inch pans, or 30 to 35 minutes for the 8-inch pans. The cakes are done when they spring back in the centers when pressed. Turn the cakes out onto wire racks to cool completely. Level each cake using a serrated knife or cake leveler.
For the buttercream: In a stand mixer fitted with the whisk attachment, mix together the butter and confectioners’ sugar. Begin on low speed until crumbly, add milk or cream to thin and then increase to high and beat for 3 minutes. Add the vanilla and beat again for another minute.
Place 2/3 cup of the buttercream in one bowl, and another 2/3 cup buttercream in a second bowl. Tint one bowl with the green food color. Tint the other bowl with the purple food color. Mix well until the colors are consistent. Transfer each color of buttercream to a separate piping bag fitted with a large closed star decorator tip. Set aside for later use.
Add the black cocoa powder to the remaining buttercream. Beat until well blended; add milk or cream if needed to thin. Add black gel food color a little at a time while mixing, until you achieve a deep black hue (about 2 teaspoons).
Place a green cake layer on a cake board. Top it with 1/2 cup of the black buttercream and spread to cover. Repeat with a purple layer. Continue until all of the layers are used. Coat the entire cake with a thin coat of the black buttercream. Refrigerate until firm, about 30 minutes.
Coat the entire cake a second time with a generous amount of the black buttercream. Smooth the top of the cake and partially smooth the sides using an offset spatula. Drag a cake comb with 1/2-inch teeth around the sides of the cake repeatedly until smooth ridges of buttercream appear. Refrigerate the cake for 1 hour.
For the syrup spray and sprinkle décors: In a small kitchen-dedicated spray bottle, combine the syrup and water. Shake well until thoroughly combined. Remove the cake from the refrigerator and spray a fine coat of syrup spray around the sides of the cake. Refrigerate the cake 30 minutes, or until very firm and slightly tacky to the touch.
Pour the sprinkles in a large baking pan. Shake to form a single even layer of sprinkles. Remove the cake from the refrigerator. Place a cake board on top of the cake. Hold the cake with each hand on the cake boards and lightly roll the cake in the sprinkles. Repeat this process until all of the ridges are coated with sprinkles. Turn the cake upright and brush away sprinkles in the recesses of the frosting using a kitchen-dedicated art brush. Press eyeball candies randomly onto the frosting ridges.
Using the bags of reserved green and purple buttercream, pipe swirls of alternating colors around the top edge of the cake. Immediately add sprinkles to the swirls. Refrigerate the cake while you make the ghost bark.
For the ghost bark: Preheat the oven to 200 degrees F.
Pour the purple melted candy onto a large parchment-lined baking sheet. Smooth with the back of a spoon. Tap the pan on a work surface to even the candy and remove any spoon trail marks.
Transfer the white melted candy to a piping bag with a tiny hole snipped in the end. Pipe large ovals of white candy onto the purple candy while it is still melted. Drag a chopstick through each candy oval to create ghostly trails and arm shapes. If the candy starts to set, place it on the baking sheet in the oven until re-melted, about 2 minutes.
Transfer the black melted candy to a piping bag with a tiny hole snipped in the end. Pipe eyes and a mouth on each ghost shape. Sprinkle Halloween nonpareils around each ghost. Refrigerate the bark for 15 minutes, or until the candy is solid.
Use a knife to score the bark around the ghost figures and break apart with your hands. Place pieces of the candy bark upright on top of the cake to decorate. Serve slices of the cake with pieces of the ghost bark.
You may have more cake batter than needed to fill the pans. Use the leftovers to make cupcakes and frost them with extra buttercream. Garnish them with scrap pieces of ghost bark.