Lightly oil two 9 in. Cake pans. Line with parchment paper, lightly oil and flour paper.
Sift the flour, baking soda, and cocoa together onto a large piece of waxed paper.
In large bowl, or bowl of electric mixer, mix butter until it is pale, yellow and fluffy. Add sugar, and continue mixing until light. The add eggs one at a time, mix well. Add vanilla, mix well.
Add dry ingredients to creamed mixture in thirds, alternating with sour cream and hot water, beginning and ending with dry ingredients.
Divide they batter between the two pans, tap pans once sharply on counter. Bake in center of oven 45-50 minutes.
While cakes bake, make icing. Combine butter, chocolate, and cream in medium size heavy saucepan over low heat. Heat until melted stirring occasionally. Remove pan from heat and whisk in confectioner's sugar until smooth. Whisk in vanilla and set icing aside until cool to lukewarm.
Cool cakes on wire rack when done. When cool, remove from pans.
When icing has cooled, pour over first cake and spread evenly. Place other cake on top. Pour and spread remaining frosting.