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1
Position a rack in middle of oven and preheat oven to 350u00b0F. Grease and flour 2 (9-by-2-inch) round cake pans.
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2
Make chocolate cake: In a glass measure, whisk together cocoa and boiling water until smooth. In a medium bowl, whisk together flour, baking soda and salt.
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3
In a large bowl, using an electric mixer, beat together butter, sugar and vanilla until well-blended (mixture will be dry). Add eggs one at a time, beating well on medium speed after each addition. Scrape down bowl. Reduce speed to low and beat in flour mixture in 3 parts alternately with sour cream, beginning and ending with flour, just until blended, scraping bowl when halfway done. Add cocoa mixture and blend. Evenly divide batter between pans.
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4
Bake cake for 30 minutes or until a tester inserted near center comes out clean. Cool cakes on racks for 20 minutes. Invert cakes onto racks; remove pans. Let cakes cool completely.
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5
Make bittersweet-chocolate frosting: Melt chocolate in a double boiler, whisking until smooth. Whisk in 2 Tbsp. water. Set aside. In another double boiler, cook sugar and egg yolks, whisking, until sugar dissolves and mixture is warm, 3 minutes. Whisk sugar mixture into chocolate mixture; stir in vanilla. Beat with an electric mixer on high speed for 3 minutes. Beat in butter, 1 piece at a time, beating until smooth.
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6
Assemble cake: Place 1 cake layer right side up on a serving plate. Spread 1 cup chocolate frosting evenly on top. Place remaining cake layer on top; frost cake all over with remaining frosting. Garnish with chocolate shavings, if desired.
