Trying to replicate a chocolate cake that has a box of pudding in it.

Fantastic--thanks so much.
(Also, by the way, how about creme fraiche instead of sour cream?)

You could try replacing part of the cocoa powder( if it is in the ingredient list) with melted or grated chocolate. Large amounts of cocoa tend to yield somewhat dry cakes. Sour ream is a great choice since it has a lot more body and fat, and will leave your cake moister than yogurt would because less of it will evaporate. The amount of leavening is also a factor; too much of it can leave your cake dry. Maybe also try brown sugar instead of white and a little bit of fruit puree (like banana for example). Pudding is mostly starch so it naturally retains liquid when it is cooked to a certain temperature; if you want to avoid boxed pudding you could try adding a tablespoon or two of pure starch of your choice, like tapioca for example.

Buttermilk helps too! Or plain yogurt instead, more moisture than sour cream.

Are you using oil or butter? I think cakes come out moister when you use oil. this recipe makes a really moist cake:
http://food52.com/recipes/2835_chocolate_bundt_cake