1. Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius). Spray two three-pound (1.36-kilogram) loaf pans with cooking spray.
2. In the bowl of your mixer, beat the eggs and sugar until light and foamy. Gradually add 3/4 cup + two tablespoons oil until fully incorporated.
3. In a separate bowl, combine the dry ingredients, including the zest. In a measuring cup, combine the milk with the water and the vanilla. Set the mixer to medium speed and alternately add the dry ingredients and the wet ingredients to the mixer, starting and ending with the dry ingredients. Do not overmix.
4. Melt the chocolate and the remaining one and a half tablespoons oil over a double boiler. Watch it carefully to make sure the chocolate doesn’t scorch and become grainy. Allow it to cool slightly.
5. Mix two to three tablespoons of batter into the chocolate. Once it’s fully incorporated, pour a third of the batter into the chocolate mixture and mix it until smooth.
6. Divide the remaining white batter between the two prepared loaf pans. Pour the chocolate batter over the batter in each pan and marbleize. Place both pans into the oven immediately and bake for 35–40 minutes, until a toothpick inserted comes out clean. (Try to avoid opening the oven door for the first 25 minutes of baking time. After 25 minutes, you can rotate the two pans in the oven for even baking.)
7. Allow to cool on a baking rack. Cut into slices and enjoy with your morning coffee! This cake will last for several days on the counter. Freezes well too.