In a 9x13 pan, combine all the wet ingredients (oil, juice and eggs). Using a fork or a whisk, mix until combined.
Add the dry ingredients (flour, sugars, baking powder) and mix until a batter forms.
Remove about 1 cup of batter from the pan and add cocoa powder, sugar and boiling water to that cup (yes, you will need one extra small bowl for this).
Drizzle the cocoa batter over the original vanilla batter, and, using a fork, marbleize it.
Bake for 1 hour, or until a toothpick inserted comes out dry.