PreparationMake crumble: Blend all ingredients in processor until nuts are finely chopped. Make cake: Preheat oven to 350°F. Butter and flour 13 x 9 x 2-inch baking pan. Whisk first 7 ingredients in medium bowl to blend. Whisk sour cream, milk and vanilla in small bowl to blend. Using electric mixer, beat butter in large bowl until fluffy. Gradually beat in both sugars. Beat in 3 eggs, 1 at a time. Beat in 1/2 cup dry ingredients. Beat in 2 eggs, 1 at a time. Beat in remaining dry ingredients in 3 additions alternately with sour cream mixture in 2 additions. Spread 3 cups batter in prepared pan. Sprinkle with crumble; press lightly into batter. Spread remaining batter over crumble to cover. Bake cake until tester inserted into center comes out clean, about 35 minutes. Cool cake in pan on rack. Make frosting: Melt chocolate in top of double boiler over simmering water, stirring until just melted and smooth. Cool to room temperature. Using electric mixer, beat cream cheese, butter and vanilla in large bowl to blend. Beat in powdered sugar in 3 additions, then cooled chocolate and cocoa. Spread top of cake with frosting. (Can be made 1 day ahead. Chill until frosting is set; cover and keep chilled. Let stand 1 hour at room temperature before serving.)
PreparationMake crumble: Blend all ingredients in processor until nuts are finely chopped. Make cake: Preheat oven to 350°F. Butter and flour 13 x 9 x 2-inch baking pan. Whisk first 7 ingredients in medium bowl to blend. Whisk sour cream, milk and vanilla in small bowl to blend. Using electric mixer, beat butter in large bowl until fluffy. Gradually beat in both sugars. Beat in 3 eggs, 1 at a time. Beat in 1/2 cup dry ingredients. Beat in 2 eggs, 1 at a time. Beat in remaining dry ingredients in 3 additions alternately with sour cream mixture in 2 additions. Spread 3 cups batter in prepared pan. Sprinkle with crumble; press lightly into batter. Spread remaining batter over crumble to cover. Bake cake until tester inserted into center comes out clean, about 35 minutes. Cool cake in pan on rack. Make frosting: Melt chocolate in top of double boiler over simmering water, stirring until just melted and smooth. Cool to room temperature. Using electric mixer, beat cream cheese, butter and vanilla in large bowl to blend. Beat in powdered sugar in 3 additions, then cooled chocolate and cocoa. Spread top of cake with frosting. (Can be made 1 day ahead. Chill until frosting is set; cover and keep chilled. Let stand 1 hour at room temperature before serving.)