Have a piece of this cake with a cold glass of milk, and you 're a kid all over again. Cake

Position rack in center of oven and preheat to 350''F. Butter three 9-inch diameter cake pans with 1 1/2-inch-high sides. Line pan bottoms with parchment or waxed paper. Butter paper. Dust pans with flour; tap out excess.

For cake: Insert Steel Knife. Place flour, cocoa, baking soda and salt in work bowl. Process to "sift, " about 5 seconds. Transfer contents of work bowl to large sheet of waxed paper.

Place eggs, then sugar into work bowl. Process until mixture is thick, about 1 minute. Add butter and vanilla. Process until mixture is fluffy, stopping to scrape down sides of work bowl, about 1 minute. Mix dry ingredients into egg mixture alternately with buttermilk in about 3 additions each, pulsing just to combine each addition.

Divide batter evenly among prepared pans (about 1% cups in each). Bake until tester inserted in center of cakes comes out clean, about 24 minutes. Cool cakes in pans on racks 10 minutes. Run small sharp knife around pan sides to loosen cakes. Turn out onto rack. Peel off paper. Cool.

For frosting: Melt chocolate and butter in heavy small saucepan over low heat, stirring until smooth. (Or to microwave: Place chocolate and butter in 4-cup microwave-safe bowl. Cook uncovered on High 1 minute. Stir well. Cook on High until almost melted, about 30 seconds longer. Stir until chocolate mixture is smooth.)

Steel Knife: Place powdered sugar, cocoa and salt in work bowl. Process to "sift, " about 5 seconds. Add chocolate mixture and vanilla. Process until just combined, about 5 seconds. Add 4 tablespoons sour cream. Process to blend, about 8 seconds. Scrape down sides of work bowl. Thin to spreadable consistency by adding enough remaining sour cream by tablespoons, processing to blend after each addition.

Place 1 cake layer on serving platter. Spread with 3/4 Cup frosting. Top with second cake layer. Spread with 3/4 Cup frosting. Top with third cake layer. Spread remaining frosting over top and sides of cake. (Can be prepared I day ahead. Refrigerate until frosting is firm, then cover and keep refrigerated. Bring to room temperature before serving.)

12 servings