I tried putting a cup of sour cream in my boxed Red Velvet cake mix and it came out of the oven nice and pretty, BUT as it cooled, it shrunk up like a wool sweater in the dryer . . . lol . . . Is there something else I can put in so keep it from shrinking when it cools. I and thinking about trying a cup of buttermilk instead of sour cream. Will the same thing happen? Do I substitute the buttermilk in place of a cup of the water the box calls for?
I don't usually have a cake shrink unless I've undercooked it. Have you cut into it to see if it's thoroughly done?
Did you use the extender recipe or just the sour cream? If just the sour cream, then you need to take a look at the extender recipe and try it. You'll need extra flour to offset the extra moisture you've added.
Give us some more info. so we can be of better help. 
Thanks for the post. Maybe that's what I did wrong and it just wasn't done enough. I did cut into it and it was pretty wet, but not "raw" so to speak. I haven't tried the cake extender yet. It never occured to me since I thought it was just to make more mix.
I have only had a cake shrink up on me if it was over cooked. How long did you let it cool in the pan? Did you let it cool completely in the pan? Since the pan is still warm when you take it out of the oven it will continue to cook the cake until it is cool also.
Or your oven may be running hot, may want to get an oven thermometer and check.
Cooking is an art, but baking is a science.
In order to obtain predictable results, it's necessary to use a balanced formula (recipe).
Changing ingredients, or adding/substracting ingredients will unbalance the formula and end results will vary.
The WASC cake recipe is a doctored mix that has box reliability and a more homemade taste.
Here's the expanded flavors version:
http://tinyurl.com/2cu8s4
Here are two cake troubleshooting charts:
http://tinyurl.com/2p5bdu
(short list from joyofbaking.com.)
http://tinyurl.com/3dmp96
(long list from Sarah Phillips of baking911.com.)
HTH
I have used the boxed Red Velvet and added one heaping tablespoon of sour cream along with the ingredients listed on the box. It always turns out fine. I have never add a whole cup before though.
Quote by @%username% on %date%
%body%