Give your marble cake an upgrade by adding a rich chocolate ganache into the mix in this Marble Sheet Cake.
Melt 1 ounce of milk chocolate in the microwave in 30-second increments until mostly melted. Stir until all chunks are fully melted. Set aside.
In a large bowl with a hand-held mixer (or in the bowl of a stand mixer) mix together the butter and granulated sugar on high until fluffy.
Add in eggs and vanilla and mix until combined.
Add in the room temperature milk and sour cream and mix until fully combined.
Add flour, baking powder, and salt to the batter and mix until JUST combined. There should be no lumps and the batter will be on the thicker side, like pancake batter.
In a separate bowl, add one cup of the vanilla cake batter. To that, add 3 tablespoons of cocoa powder, 1/2 teaspoon baking soda, and the 1 ounce of chocolate that was melted in the beginning.
Stir well. (The chocolate batter will be very thick.)
In a small bowl add the 2 ounces of semi-sweet chocolate and heavy whipping cream.
Heat for 1 minute in the microwave and then stir for about a minute. The mixture should come together and be thicker and glossier when done.
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.