Preheat the Oven:
Preheat your oven to 350°F (175°C). Grease and line two 8-inch or 9-inch cake pans with parchment paper.
Prepare the Dry Ingredients:
In a large bowl, sift together the flour, black cocoa powder, baking powder, baking soda, and salt.
Mix the Wet Ingredients:
In another bowl, whisk sugar, oil, eggs, and vanilla until smooth. Gradually add the buttermilk, mixing until combined.
Combine and Add Color:
Gradually add the dry ingredients to the wet mixture, alternating with the hot water or coffee. Stir until smooth. If desired, add black gel food coloring for an extra dark hue.
Bake the Cake:
Divide the batter evenly between the prepared pans. Bake for 25–30 minutes or until a toothpick inserted in the center comes out clean. Cool in the pans for 10 minutes, then transfer to a cooling rack to cool completely.
Prepare the Frosting:
Beat butter until creamy. Gradually add powdered sugar, mixing well. Stir in vanilla and heavy cream until smooth and spreadable. Add black food coloring if desired.
Assemble the Cake:
Spread frosting between the cooled cake layers. Frost the top and sides of the cake, smoothing with an offset spatula.
Decorate:
Add spooky decorations like black sprinkles, candy spiders, or edible glitter to enhance the theme.