Preheat oven to 350 degrees F (175 degree C).

Grease and line three 7-inch, three 6-inch, and two 4-inch cake pans.

In a large bowl, whisk together the vegetable oil, eggs, buttermilk, white vinegar and vanilla extract.

Add granulated sugar, and whisk for 30 seconds to help the dissolve the sugar.

Mix in the cocoa powder and baking soda, and whisk until no clumps remain.

Add green gel food coloring, and mix until the batter is evenly colored a deep shade of green.

Mix the flour into the batter in two installments.

Divide the batter between the prepared pans, filling them about 3/4 inch high with batter.

Bake for 27-29 minutes, or until a toothpick comes out clean.

Remove pans from oven, and allow them to cool in the pans for 10 minutes.

Carefully flip the layers out of the pans, and place on a wire rack to finish cooling.

Level the cake tops with a serrated knife once they're fully cooled.

Beat the butter on a medium speed for 30 seconds with a paddle attachment, until smooth.

Add in the vanilla and salt, and beat on low until the ingredients are fully incorporated.

Slowly mix in the powdered sugar on a very low speed, 1 cup at a time. Alternate with small splashes of cream. 

If the frosting is too thick, add in additional cream (1 tablespoon at a time).

If the frosting is too thin, add in more powdered sugar (1/4 cup at a time).

Color 3/4 of the frosting green, and divide the remaining frosting between two bowls. Color one a deep shade of purple (zombie t-shirt), and the other a pinkish red color (zombie brains).

Cover each bowl of frosting with a piece of plastic wrap, to prevent a crust from forming until you're ready to use it.

Heat heavy cream in a heatproof bowl in the microwave for 45 seconds, until it's just beginning to bubble.

Gently pour white chocolate chips into the heavy cream, making sure they are fully covered with cream.

Allow mixture to sit for 1 minute.

Stir slowly until the cream and white chocolate are combined.

Some bits of chocolate may not be fully melted yet. Heat the mixture again for 20 seconds, and stir. Repeat as necessary until all the chocolate bits are fully melted and incorporated.

Add in the red gel food coloring, if desired. Allow to cool until desired viscosity is reached (barely warm to the touch).

Stack and frost cake layers on a greaseproof cake board, using a dab of frosting to help stick the first cake layer to the board. For the bottom tier, stack the cake layers from bottom to top in this order: 7 inch, 7 inch, 6 inch, 4 inch.

Add an even layer of buttercream between each cake layer.

Add a thin coat of frosting around the cake, to fully cover the cake layers. Smooth using an acetate sheet, then add in a central dowel, and three trimmed bubble tea straws to support the weight of the zombie's head.

Chill the cake in the fridge (30 minutes) or freezer (10 minutes) until the frosting is firm to the touch.

Press two bubble tea straws into the sides of the cake, to be the zombie arms. Cover with green frosting, then add purple frosting over the top of the green frosting to create sleeves.

Add a second, thicker layer of purple frosting to the cake, and smooth using an acetate sheet.

Chill in the freezer for an additional 10 minutes.

While the body chills, assemble the zombie head on the 4 inch, greaseproof cake board. Be sure to cut a hole in the center of the board that is large enough to fit over the central support used in the zombie's body.

Stack the cake layers from bottom to top in this order: 6 inch, 7 inch, 6 inch, 4 inch.

Frost with an even layer of buttercream between each cake layer, and cover with a crumb coat. Chill in the freezer for 10 minutes.

Add a thin second layer of of pink frosting to the top 1/4 of the cake. Place the remaining pink frosting into a piping bag and cut an opening about 3/4 cm wide.

Pipe squiggly lines vertically on top of the cake, to look like zombie brains.

Add a second, thicker layer of frosting to the bottom part of the cake, working the frosting up toward the brain.

Leave a jagged edge when the green frosting meets the pink frosting.

Smooth using an acetate sheet.

Add fondant facial features, and drizzle ganache over the pink frosting and over the edge of the green frosting.

Chill zombie head for 10 minutes in the freezer.

Remove zombie head and body from freezer, and carefully lower the zombie head onto the central dowel.

Slowly lower the head with a large offset spatula, until it's resting on the body.

Fill the gap between the two tiers with some additional green frosting.

Use a knife to cut some holes in the zombie's shirt, by scraping away some of the purple frosting to expose the green frosting underneath.

Then cut in and enjoy!