To make a marble cake, you hold back a small amount of the vanilla cake batter and add chocolate or cocoa powder to it and then drop it into the cake batter in the pan in small blobs all over. Then take a butter knife and swirl through the batter to distribute the chocolate batter, but not mix it completely together.
or you can also make up a white cake and chocolate cake (or any other two flavors) and drop in the pan and swirl like mentioned above. I find my customers prefer this as my chocolate cake has more chocolate flavor instead of just adding chocolate to my white cake batter.
I add Hershey's chocolate syrup to a small amount of vanilla/yellow cake batter then swirl in with a knife. It does take quite a bit of syrup though but it tastes yummy 
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