Preheat your oven to 350°F (175°C). Grease and flour three 8-inch round cake pans to ensure easy removal after baking.
In a large mixing bowl, combine sifted flour, cocoa powder, baking powder, baking soda, and salt. Whisk until well mixed to ensure even distribution of dry ingredients.
In another bowl, mix the buttermilk, vegetable oil, eggs, and vanilla extract until smooth and combined. This creates the wet mixture for your batter.
Add the wet mixture to the dry ingredients gradually, mixing on low speed. Once combined, stir in the hot water slowly to thin the batter; it will be quite liquid.
Divide the batter evenly among the prepared cake pans. Smooth the tops with an offset spatula for an even bake.
Bake in the preheated oven for about 25 minutes, or until a toothpick inserted into the center comes out clean. Then, remove from oven and let the cakes cool in their pans for 10 minutes.
Transfer the cakes to a wire rack and let them cool completely before frosting. This prevents melting and makes layering easier.
Prepare the chocolate frosting by beating unsalted butter until creamy. Add powdered sugar gradually, then stir in cocoa powder. Mix until smooth and fluffy, adding milk as needed to reach a spreadable consistency.
Place the first cake layer on your serving plate and spread an even layer of frosting on top. Repeat with the second layer, then smooth the outer surface of the assembled cake with a spatula.
Apply a final thin layer of frosting over the entire cake to seal in crumbs. Chill in the fridge for 20 minutes if needed to set the frosting firmly.
Fill a piping bag fitted with a round tip with white icing. Starting at the center of the top of the cake, pipe concentric circles, then connect the circles with radial lines to create a web pattern, covering the entire top surface.
Carefully add more web lines, extending from the center to the edges and intersecting existing lines, to enhance the web effect. Observe the pattern come together with each piping stroke.
Once finished, chill the decorated cake for about 15 minutes to set the web pattern. Slice and serve to reveal the beautifully spooky design with rich chocolate layers.
