Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans, then set aside.
For the chocolate layer, whisk together sifted flour, cocoa powder, baking soda, and salt in a bowl. In another bowl, combine buttermilk, oil, eggs, and vanilla. Gradually mix the wet ingredients into the dry until smooth.
Pour the chocolate batter evenly into the prepared pans. Bake for about 30 minutes until a toothpick inserted in the center comes out clean. Cool in pans for 10 minutes, then transfer to a wire rack to cool completely.
For the vanilla layer, whisk together sifted flour, baking powder, and salt. In another bowl, mix milk, oil, eggs, and vanilla. Combine the wet and dry ingredients and stir until smooth.
Divide the vanilla batter evenly into two prepared pans. Bake at 350°F (175°C) for 25–30 minutes until a toothpick tests clean. Allow to cool completely on a wire rack.
While the cakes cool, beat softened butter and powdered sugar together until fluffy. Add milk gradually to reach a smooth, spreadable frosting. Tint acrylic icing white for decoration.
Once cakes are cooled, carefully level the tops if needed. Spread a layer of frosting on the chocolate layer, then top with the vanilla layer. Repeat with the second layers and crumb-coat the entire cake, smoothing the surface.
Apply a final thick layer of frosting over the cake, smoothing with a spatula. Chill the cake for 30 minutes to set the frosting.
Using a piping bag fitted with a fine tip, draw concentric circles on the top of the cake with white icing. Then, use a thin toothpick or skewer to draw lines from the center outward to mimic a spider web.
Decorate the web further by adding small edible spiders or web accents if desired. Place the completed cake on a serving platter.
This Spider Web Cake is ready to serve! Slice gently to reveal the layered chocolate and vanilla interior, and enjoy the spooky, delicious treat.