I don't know why it's so good (doesn't seem like anything special from looking at the recipe) but it's the one thing that will go on a table loaded with good desserts! Originally found in the Silver Palate
Pour boiling water over chocolate and butter; let stand until melted.
Stir in vanilla and sugar, then whisk in egg yolks, one at a time, until blended well.
Mix soda and sour cream and whisk into chocolate mixture.
Mix flour and baking powder well, and add to batter, mixing thoroughly.
Beat egg whites until stiff but not dry. Stir in a quarter of the egg whites thoroughly into the batter. Scoop the remaining egg whites on top of the batter and gently fold together.
Pour batter into prepared pan. Set on the middle rack of the oven and bake for 40-50 minute or until the edges have pulled away from the pan and the cake tester inserted comes out clean. Cool in pan for 10 minutes; unmold and cool completely before frosting.
To Make the Frosting:
Place all ingredients in a heavy saucepan and whisk until smooth over low heat. Cool slightly; add more sugar if necessary. Spread on cake while frosting is still warm.