Preheat the oven to 350 degrees and spray 2- 8 or 9″ pans with baking spray. I used 9″ but 8″ will also work!
In a medium bowl, combine the cake mix, buttermilk, oil, and eggs until fully mixed. Divide the cake mix between the two pans and bake according to the package instructions. Let it cool in the pan for 10-15 minutes and then move to wire racks to cool completely.
In a mixing bowl with a hand mixer or a stand mixer, beat the butter until smooth. Add in half of the powdered sugar and mix. Add in the rest of the sugar, vanilla, and heavy cream and then mix until smooth.
Divide the frosting into 3 bowls. In one bowl, put 3/4 c frosting and mix I. 1/3 c fluff. In another bowl measure 3/4 c frosting and tint it purple. If you’d like to add the light brown swirl like I did, tint 80% purple and 20% light brown. Put the remaining frosting in the last bowl and mix in the cocoa powder.
When you’re ready to decorate the cake, place it on a cake board onto a turn table. Place the first cake layer down and spread the marshmallow buttercream on top. Then flip the other cake layer over (so that the bottom faces up) and place it on top of the first layer.
Spread the chocolate frosting on the top and sides of the entire cake. This will be the crumb coat. Basically it is sort of a primer for the second layer of buttercream. Place it in the fridge for 30 minutes or until it’s cool. Spread the second layer or frosting on the cake and refrigerate again for another 30 minutes.
In a small microwave safe bowl, heat the heavy cream (1/3 c) for one minutes. Remove it from the microwave and add the chocolate chips. Let it stand for 30 seconds and then whisk it until smooth. Let it cool for about 5 minutes or place it in the fridge. You want it to be warm, not hot so that it doesn’t pour off of the cake.
Once you’ve removed the cake from the fridge, spoon the ganache on top, letting it cascade lightly over the edges. Refrigerate it again for about 10 minutes or until set.
Slice the cake with a large knife into 8 even slices.
Using a piping bag with a 1M tip, tip swirls of buttercream along the base of the cake. It should swirl along the entire outside bottom edge of the cake. Place the rest of the purple frosting in another piping bag fitted with a small star tip. Pipe stars of frosting (or swirls) around the top border of each cake, stopping between each slice so that you ca. see where they separate.
If desired, add the brown swirls in the center of the purple swirls/stars you just piped.
Add a sprinkle of cereal underneath each purple buttercream border. Add the gummy vampire teeth and underneath that, the chocolate sprinkles.
If doing so, add the chocolate bats to the perimeter of the cake. Each slice should get a bat, and make sure they are centered and “sitting” on the buttercream. This way they won’t slide down the cake.