I recently made a chocolate cake, and needed more than just the one box mix (but all I had was one), so I added the extender recipe with the sour cream. I also added a bit of Chocolate Lorann oil, instead of extract.
Well, the cake baked up beautifully. It rose up over the pan, so I was able to level it easily. It was more dense than a normal box mix, but it was still very moist and soft.
Oh, it's the cigarello cake in my photos, so you can see how well it baked up. I filled two 9 inch cake pans with it. I was afraid it wouldn't be moist because it was dense, but it was great. My husband took it to work for his coworkers to finish off, and they all just loved it.
This is the recipe I used. It was wonderful.http://www.cakecentral.com/cake_recipe-1977-cake-mix-extender.html
I used the ingredients there, but I just added everything together in my mixer bowl. I put the box mix in, then all the ingredients called for on the box mix, then I added all ingredients in the extender recipe. Turned on the mixer, poured in the pans after well mixed, then just baked it in the pans as stated on the box directions.
I hope this helps,