I found the recipe of the cake some time ago on Duncan Hines. While the icing comes from one of the books of BHG. It is the best marble cake I ever had. Made on several occasions for company, and they couldn't stop eating it.
Microwave chocolate with 20-30 sec intervals. Stir and repeat until most of the chocolate is melt but not all (usually takes 1 minute). I usually do not let it melt completely, which creates a nice effect in the cake.
Combine cake mix, eggs, vanilla extract, sour cream, vegetable oil, sugar and water. Beat on high for around 3 minutes.
Stir third of the batter into the melted chocolate, mix well.
Spread half of the vanilla batter in the pan, then the chocolate batter and then the rest of the vanilla batter. Swirl the batters with a butter knife to create the marble effect.
Bake for 35-40 minutes, cool for 20 minutes in pan, remove from pan and cool completely.
For the icing: melt the chocolate and butter (you can do this in the microwave as well, but be careful).
Cool the mix for 5-10 minutes, add sour cream and then the powdered sugar 1 cup at a time.
Spread the icing on the cake when it has cooled completely.