Dee, I’m not sure if I have the magic answer on how to do this. I’ve baked the marble cake and my sense of it is that the cake is already holding up the heavy melted chocolate mixture, so that it doesn’t have as much structure and support as a regular layer cake.
There is probably a way to make it work, and if I were going to try it my first attempt would include these details:
-Scale the recipe to fit your pan’s volume, then reduce baking powder by 20% after scaling.
-When placing the heavier chocolate mixture on the cake, I would put less or even avoid altogether the very middle, to help it keep from sinking.
-Use cake strips
-Use an inverted flower nail or heat core in the center
-Make sure the oven is not too cool.
-Use a thermometer and make sure the center makes it to at least 190F before removing it from the oven.
If you do try it, please report back.