Want to make a marble birthday cake using the batters from my favorite chocolate cake recipe and vanilla cake recipe. Will this work? Is there a better way?
It should work. I think marble cakes work best when the two batters are of similar weight, though...so if your chocolate cake is heavy, for instance, and your vanilla cake is super light, it might not give you the results your looking for. What I generally do is split my vanilla butter cake recipe in half and turn on into chocolate and leave the other vanilla. Since I do everything by weight, I just split the dry ingredients in half and prepare a whole batch of wet ingredients before evenly dividing it between the dry. It sounds complicated, but its actually quite easy and I'm not making two cake batters. Its not my favorite chocolate cake recipe to use on its own, but it has a much better synergy with the vanilla and I know that the marble pattern is going to maintain and stay separate. It makes a better marble cake, which is what you want.
My grandma used to do that, so I know it can work. Sadly I don't have her recipe and honestly have not thought about this cake in years. She was one who never used a recipe, and used coffee cups to measure with. It's probably been 30 years since I've had it. I do recall it was in a bundt pan and not a fluffy, light cake. It was heavier, but not all the way to a pound cake if that makes sense. Maybe you can post if you get something that works. I'd love to try it!
I think you would be better off to add cocoa to 1/3 of your vanilla cake recipe. Unless you are certain the two recipes you noted above are of the same weight.