Prepare Your Equipment

1. Preheat your oven to 350°F (175°C).
2. Grease a standard loaf pan and line it with parchment paper for easy removal.

2. Mix the Dry Ingredients

1. In a medium bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.

3. Combine the Wet Ingredients

1. In a large bowl, beat sugar and vegetable oil until smooth.
2. Add eggs one at a time, mixing well after each addition.
3. Stir in vanilla extract, orange zest, orange juice, and buttermilk.

4. Create the Batter

1. Gradually fold the dry ingredients into the wet mixture, mixing until just combined.
2. Gently fold in chocolate chips.

5. Create the Marble Effect

1. Divide the batter evenly into two portions. To one portion, add 1 tablespoon extra cocoa powder and mix until combined.
2. Alternate spoonfuls of each batter into the prepared loaf pan.
3. Use a skewer or knife to swirl the batters together for a marble effect.

6. Bake the Cake

1. Bake the loaf in the preheated oven for 50-60 minutes, or until a toothpick inserted into the center comes out clean.
2. Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

7. Slice and Serve

1. Slice the cooled cake into even pieces and enjoy with your favorite beverage or as a standalone treat.