Preheat your oven to 350°F (175°C). Grease and flour two 8-inch round cake pans to prepare for baking.
In a large bowl, whisk together the sifted flour, cocoa powder, baking powder, baking soda, salt, and sugar for an even dry mixture.
Add eggs, milk, vegetable oil, and vanilla extract to the dry ingredients. Use a hand mixer or whisk to blend until smooth and combined, about 2-3 minutes.
Slowly pour in the boiling water while mixing on low speed. Continue mixing until the batter is thin and has a glossy appearance, about 1 minute.
Divide the batter evenly between the prepared pans and smooth the tops with a spatula.
Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Let the cakes cool in pans for 10 minutes, then transfer to a wire rack to cool completely.
While the cakes cool, prepare the web icing by mixing powdered sugar, corn syrup, vanilla extract, and a few drops of black food coloring in a bowl. Stir until smooth and thick enough to pipe but still pliable.
Once the cakes are completely cooled, place one layer on your serving plate. Spread a thin, even layer of frosting or buttercream over the top, then gently place the second cake layer on top.
Using a piping bag fitted with a small round tip, pipe concentric circles of black web icing over the top of the cake to create the spider web pattern.
Starting from the center, draw lines outward from the middle to mimic the spokes of a spider web, using a toothpick or skewer to drag the icing into the web design for a more authentic look.
Finish by adding some decorative spiders or additional web details if desired. Chill the cake for at least 30 minutes to let the web set.
