Preheat your oven to 350°F (175°C) and grease your cake pans. In a large bowl, whisk together the cake flour, cocoa powder, baking soda, and salt.
In another mixing bowl, beat together the unsalted butter and granulated sugar until light and fluffy, about three minutes.
Add in the eggs one at a time, mixing well after each addition. Stir in buttermilk, vegetable oil, green gel food coloring, vanilla extract, and white vinegar.
Gradually add the dry mixture into the wet ingredients until just combined, being careful not to overmix.
Divide your batter evenly between prepared pans and bake for about 28 minutes, or until a toothpick comes out clean. Allow cakes to cool in pans for about ten minutes before transferring to wire racks.
Prepare the cream cheese buttercream frosting by beating together unsalted butter and cream cheese until creamy. Gradually add powdered sugar and salt, mixing until fluffy. Add heavy whipping cream for extra smoothness.
Level off any domed tops of your cakes if necessary. Place one layer on your cake board and spread an even layer of frosting on top before adding another layer of cake. Repeat until all layers are stacked.
Apply a thin layer of frosting around the sides and top of the cake as a crumb coat. Chill briefly before applying a thicker layer of frosting for a smooth finish.
Use colored gel food coloring and piping bags to create spooky decorations like Frankenstein's features.