A fluffy whipped chocolate frosting is the crowning glory to this three-layer, light-as-air cake.
1. Adjust oven rack to top of lower third of oven. Place sugar in small heavy-duty plastic food-storage bag; seal; freeze 20 minutes.
2. Heat oven to 350 degrees F. Grease three 9 x 2-inch round metal cake pans. Line bottoms with waxed paper; grease paper; flour pans and paper.
3. Pour boiling water over cocoa in bowl; stir to dissolve. Stir in buttermilk and vanilla until smooth; mixture should be cool.
4. Place large mixing bowl and beaters in freezer 5 minutes.
5. Sift together flour, baking powder, baking soda and salt onto waxed paper. Repeat 2 more times.
6. To cream, beat butter in chilled bowl with beaters 3 minutes, until light in color. With mixer running, add chilled sugar in stream. Beat 3 to 4 minutes, until fluffy. Add eggs, one at a time, beating 30 seconds after each; beat 1 to 2 minutes, until airy.
7. Fold in two-thirds of flour mixture; fold in cocoa mixture. Repeat with remaining flour mixture. Pour into prepared pans.
8. Bake in 350 degrees F oven 25 minutes or until centers spring back when touched. Cool cakes in pans on racks 10 minutes. Turn cakes out to cool. Peel off waxed paper.Meanwhile, prepare Chocolate-Cream Frosting:
9. Melt semisweet chocolate, unsalted butter and 1/4 cup of the heavy cream in saucepan over very low heat; stir until smooth. Cool. Beat the 1 cup cream, confectioners sugar and vanilla in bowl just until soft peaks form; do not overbeat to prevent graininess. Fold third of whipped cream into chocolate mixture just until combined. Fold chocolate mixture into remaining whipped cream just until combined.
10. Place cake layer on serving plate. Spread top with one-third of frosting. Place second layer on top; spread with third of frosting. Repeat with remaining layer and frosting. Garnish. Refrigerate until serving.