Preheat oven to 350 degrees F. Grease a 9x5 inch loaf pan and line with a strip of parchment paper.
Make the chocolate paste first. In a small bowl, mix together the cocoa powder and oil until smooth. If the mixture appears too stiff, you can add a tiny bit more oil.
Whisk the flour, baking powder, baking soda and salt together in a bowl and set aside.
In the bowl of a stand mixer (or in a mixing bowl using a handheld), beat the oil and sugar until flight and fluffy. Beat in the eggs, vanilla, orange juice and water.
Add the flour mixture and stir until blended.
When fully blended, spoon out about ⅔ cup of the batter and put it in another bowl. This will be your white batter.
To the batter left in the mixing bowl, add the cocoa mixture and beat until smooth. If using grated chocolate or chips, add them too.
Spoon batter into pan, starting with vanilla and alternating the two batters in any pattern.
Bake the loaf for about 35 to 40 minutes.
Let cake cool in pan on a rack 20 minutes. Run a thin knife around edges of cake, then turn from pan and let cool completely. When cool, dust with powdered sugar or make an orange glaze by mixing ½ cup sifted confectioners' sugar with about 3-4 tablespoons orange juice.
