Preheat oven to 350°F (175°C). Grease and line two 9-inch round cake pans.
In a large mixing bowl, whisk together flour, sugar, cocoa, baking soda, baking powder, and salt.
Add eggs, buttermilk, oil, and vanilla. Mix until smooth.
Stir in the hot water or coffee until combined. Batter will be thin.
Divide batter evenly between pans and bake 30–35 minutes, or until a toothpick inserted in the center comes out clean.
Let cakes cool completely before frosting.
For frosting: beat butter until creamy. Gradually add powdered sugar, milk, and vanilla. Adjust consistency with more sugar or milk. Tint with food coloring as desired.
Spread frosting between layers and over the outside of the cake.
Decorate with spooky elements—create a ghost cake, witch cake, or coffin cake design using your decorations.