Pour the vanilla cake into a 10x15-inch baking pan coated with non-stick spray.
Place the chocolate cake batter into a pastry bag and cut off about an inch from the top.
Pipe the chocolate batter onto the vanilla batter in a zig-zag pattern, starting at one end. (You may also dollop the chocolate batter in as much of the pattern as you can if you don't have a pastry bag.)
Take a kitchen knife and do another zig-zag pattern the opposite way (horizontally).
Now pour the ganache in the same loose zig-zag swirl pattern and run the knife through one more time.
Bake for 16-22 minutes, or until when pressed, the center of the cake bounces back. If you insert a toothpick, it should be removed with some crumbs, but not wet batter.