Gumdrops stud the surface of this domed cake. Icing is piped out in starburst dots. You can't go wrong -- just start piping at the bottom and work your way up.
One-Bowl Chocolate Cake: Preheat oven to 350 degrees. Butter two 8-inch round cake pans and one 8-inch stainless-steel bowl, and line bottoms with parchment; butter parchment, and dust with cocoa.
Into the bowl of an electric mixer fitted with the paddle attachment, sift cocoa, flour, sugar, baking soda, baking powder, and salt. With the mixer on low, stir in eggs, 1 1/2 cups warm water, buttermilk, vegetable oil, and vanilla until smooth, about 3 minutes.
Divide batter among prepared pans and bowl. Bake, rotating once, until tester inserted in center comes out clean, 35 to 45 minutes for 8-inch layers and 50 to 55 minutes for bowl.
Let cakes cool on wire rack for 20 minutes, then remove and cool completely, right side up on rack. Cakes can be made in advance and stored up to 1 day in refrigerator. They can be stored 1 week in the freezer.
Swiss Meringue Buttercream: Beat butter with electric mixer until fluffy and pale. Transfer to small bowl.
In double boiler over simmering water, whisk sugar and egg whites until warm and sugar is dissolved, 2 to 3 minutes. Transfer to clean bowl of electric mixer; beat on high with whisk attachment until fluffy and cooled, about 10 minutes.
Reduce mixer to medium-low; add butter a scant 1/4 cup at a time, beating well after each addition. Mix in vanilla.
Switch to paddle attachment; beat on lowest speed 3 to 5 minutes. Leave at room temperature if using same day. Or store airtight in refrigerator up to 3 days. Bring to room temperature; beat until smooth.
With serrated knife, trim the tops of layers that were baked in cake pans to be level. Brush excess crumbs away. Place bottom layer on a cake round, and use an offset spatula to evenly spread 1 1/4 cups buttercream on top. Trim the uneven top away from the layer that was baked in the bowl.
Add domed layer, cut side down, and spread 1 1/2 to 2 cups buttercream in thin layer over entire cake to seal in crumbs. Transfer assembled and crumb-coated cake to refrigerator, and chill 30 to 40 minutes, or until buttercream is firm. For best piping consistency, keep the remaining buttercream at room temperature.
Transfer cake to rotating cake stand for easy decorating, or place on inverted cake pan to elevate. Fill pastry bag, fitted with #6B tip, with remaining buttercream. Starting at bottom of cake, pipe 1 1/4-inch stars around the base, then add stars in horizontal rows up sides and across top to completely cover cake.
Place one gumdrop in the center of each buttercream star, creating a colorful pattern around the cake. Chill completed cake at least one hour before serving so buttercream is firm and set.