Preheat oven to 325 degrees. Grease a long loaf pan or 12-cup tube pan.
In the bowl of a stand mixer, mix together cream cheese and butter until smooth. Add sugar to butter mixture and cream until light and fluffy. Add eggs, two-at-a-time, and vanilla. Mix well. Mix in flour until well combined. Pour 3/4 of the batter into loaf pan.
Add cocoa to the remaining 1/4 of batter and mix well. Dollop the chocolate batter on top of the plain batter. With a knife swirl batter to form a marble look.
Bake for 1 hour and 15 minutes or until cake tester inserted near center comes out clean. Cool on wire rack before turning out onto serving platter.