Devil's Food Cake with Chocolate Spider's Web

Preheat oven to 350 ° F. Grease three 9-inch round cake pans and line each bottom with a round of wax paper. Grease paper and dust pans with flour, knocking out excess flour. In a bowl whisk boiling water into cocoa until smooth and whisk in milk and vanilla. In a large bowl with an electric mixer beat together butter and sugars until light and fluffy and beat in eggs, 1 at a time, beating well after each addition. Into another bowl sift together flour, baking soda, and salt and add to egg mixture in batches alternately with cocoa mixture, beginning and ending with flour mixture and beating well after each addition. Divide batter among pans, smoothing tops, and bake in upper and lower thirds of oven, switching position of pans halfway through baking, 20 to 25 minutes, or until a tester comes out clean and layers begin to pull away from sides of pans. Cool layers in pans on racks 10 minutes and turn out onto racks to cool completely. Cake layers may be made ahead and kept, wrapped well in plastic wrap, at room teperature 2 days or frozen 1 week.