1. In small bowl, combine 2/3 cup sugar, the cocoa and warm water; set aside.
2. Heat oven to 350 degrees F. Lightly butter three 8-inch round cake pans. Line bottoms with parchment paper; butter paper. Set aside.
3. In a large bowl, whisk together flour, baking powder, baking soda and salt. In a mixer bowl, beat together remaining 2 cups sugar and the butter on medium speed until light and fluffy, about 1 minute. Reduce speed to low and beat in extract. Add eggs, one at a time, beating well after each addition and scraping down sides of bowl. Alternately beat in flour mixture and milk, beginning and ending with flour mixture, scraping down sides of bowl, until combined.
4. Transfer 3 cups batter to a bowl and stir in cocoa mixture. Evenly spoon dollops of white and cocoa batters into prepared pans. Using a knife, swirl batters to marbelize. Bake until a toothpick inserted in center comes out clean, 40 to 45 min. Transfer pans to wire racks and let cool 15 min. Remove cakes from pans, peel off paper and place, right side up, on racks; let cool completely.
5. Transfer 1-1/2 cups buttercream to a decorating bag fitted with a #5 plain tip and set aside.
6. Slice domes off cakes to level. Place 1 cake layer on a serving plate; spread just to edges with 1-3/4 cups buttercream. Cover with second layer and spread with another 1-3/4 cups buttercream. Top with third layer. Frost top and sides, smoothing with a spatula. Using reserved piping bag, pipe dots around cake base. Refrigerate until set, 20 to 25 min.To decorate::
7. Fit a decorating bag with a #2 plain tip. Cut bottom from black icing tube and transfer to decorating bag.
8. Pipe a spiderweb design on top and down sides of cake in varying lengths. To pipe spiders, hold icing tip in one place while squeezing tube to form a thick, round body. Pipe 8 thin legs out from each body. (Decorated cake can be made ahead. Store in a sealed cake container in the refrigerator up to 1 day.) Makes 16 servings.
1. In a medium heatproof bowl set over simmering water, combine sugar and egg whites; stir until hot and sugar is dissolved, about 5 minutes. Transfer to a mixer bowl and beat to firm peaks, about 8 minutes. While mixer is running, add butter, a few tbsp. at a time, until completely combined. Reduce speed to low; beat in chocolate and extract. Use immediately. (Can be made ahead. Store in an airtight container in the refrigerator up to 2 weeks. Bring to room temperature, then beat on low speed until smooth before using.) Makes 10 cups.