For food cake:

Preheat oven to 350F. Grease and flour two 8-inch-diameter cake pans with 1 1/2-inch-high sides. Whisk first 5 ingredients in large bowl to blend. Sift in flour, cocoa and baking soda. Stir to combine. Divide batter between prepared pans. Bake until toothpick inserted into centers comes out clean, about 25 minutes. Cool in pans on racks 10 minutes. Turn cakes out onto racks and cool completely. (Can be prepared 1 day ahead. Wrap cakes separately in plastic wrap and store at room temperature.)

Place 1 cake layer on plate. Spread with 1/2 cup frosting. Top with second cake layer. Spread top and sides of cake with remaining frosting. (Can be prepared up to 8 hours ahead. Cover with cake dome and let stand at room temperature.)

For Frosting:

Melt unsalted butter and chocolate in heavy small saucepan over low heat, stirring constantly until smooth. Transfer to large bowl. Whisk in cocoa powder and vanilla. Whisk in 1 cup sugar and 1/3 cup sour cream. Whisk in remaining 1 cup sugar. Gradually whisk in enough of remaining sour cream to form spreadable frosting.