Position a rack in middle of oven and preheat oven to 350°F. Grease and flour 2 (9-by-2-inch) round cake pans.
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.
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.
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.
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.
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.
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