Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.

In a mixing bowl, combine flour, sugar, cocoa powder, baking powder, baking soda, and salt.

Add egg, vegetable oil, buttermilk, and vanilla extract. Mix until smooth.

Stir in boiling water until well combined. Pour batter into prepared pans.

Bake for 25 minutes or until a toothpick inserted comes out clean. Cool in pans for 10 minutes, then transfer to a cooling rack.

In a mixing bowl, beat the softened butter until creamy.

Gradually add powdered sugar and heavy cream, mixing until smooth.

Add vanilla extract and green food coloring, mixing until desired color is achieved.

Once the cakes are completely cool, place one layer on a serving plate.

Spread a layer of frosting on top, then place the second cake layer on top.

Frost the top and sides of the cake with the green frosting.

Use chocolate chips to create eyes and red gel icing for blood decoration.