For the cake batter: Preheat your oven to 350 degrees F (180 degrees C). Spray a 13x9-inch sheet cake pan with cooking spray and line the bottom with parchment paper.
Using a sifter, sift the dry ingredients (flour, sugar, baking powder, baking soda, and cocoa powder) in the bowl of an electric mixer with a paddle attachment and mix to combine. Then add the wet ingredients in (mayonnaise, oil, buttermilk, hot water, and vanilla) and beat just until combined. Add the eggs and beat on low for almost a minute.
Pour the cake batter into your prepared baking pan. Bake for about 45-50 minutes. Test for doneness using a toothpick in the center of the cake.
After you allow your cake to cool for a few minutes, invert it onto a rack to come to room temperature. Remove the parchment paper from the bottom of the cake and spread the buttercream frosting on top. Decorate as desired.
For the Halloween graveyard cake, I used chocolate cookies and wrote RIP with melted white melts but you can use white chocolate chips instead. I used sprinkles and Wilton sugar ghosts plus mini pumpkin candies. I also drew a spider web with the melted candy melts and topped with plastic spiders.
For the frosting: Beat the butter in a large bowl until creamy, about 2 minutes. Then, add the confectioners’ sugar, cocoa powder, heavy cream, and vanilla extract.
Beat on low speed for 30 seconds. Increase the speed to high, beating for 1 minute. Add 1/4 cup more sugar or cocoa powder if frosting is too thin. But if it is too thick, add a tablespoon of cream.