What is the best way to make a marbled cake? Ex: I want to make a batter of French Vanilla, then I'd like to marble some chocolate through it.
Can I just add Hershey's baking cocoa to some of the vanilla batter to use for the marbling? And...what is the best method to marble the choc into the vanilla for a good thorough marbling turnout after baking?

I usually make whichever cake I want to marble (ie. vanilla, white, yellow) and then take out about cup if it's a single recipe and add 2 oz of melted semi-sweet chocolate to it. Then fill your pans with your vanilla batter. Drop tablespoon fulls of the batter around the top and then take a knife and swirl it. There's really no wrong way to marble.

I had a "marbling" disaster last weekend. The scratch recipe I used had a very thick batter. I reserved about a cup of the batter and added "unsweetened" chocolate. Since the batter was so thick when I added the chocolate mixed batter by the spoonfuls it was so thick it wouldn't swirl very good. So when it came out there were more unsweet chocolate chunks in the cake. I think it may depend on your batter consistency. With my particular recipe I used I would have been better off using "semi sweet" chocolate and directly swirling it into the batter.

According to the Pillsbury experts, if you're using a cake mix, divide the batter in half and use 2 TB cocoa powder to it. In the pan alternate adding yellow and choc batter, then run a knife through it gently to swirl. I'm sure cooled melted semi sweet chocolate would be fine too. Good luck. I know I learned from my disaster.