Preheat oven to 350 degrees; grease and flour 10-12 cup bunt pan or line two 6 cup loaf pans with parchment.
In a medium bowl, whisk the cocoa, sugar, and water until well blended. Set aside.
In a large mixer bowl (with the whisk attachment), beat sugar, oil, vanilla, and salt until well blended. Add the eggs one at a time, beating well after each addition. Continue to beat until the mixture is thick and pale, 3-5 minutes. Stop the mixer and add 1/3 of the flour mixture. Beat on low speed, scraping the bowl as necessary, just until incorporated.
Add 1/2 of the milk and beat just until it is blended. Repeat with another third of the flour, the remaining milk, and then the remaining flour.
Measure and stir 2 cup sof the batter into the cocoa mixture. Stir the orange zest into the plain batter. Pour 1/3 of the orange batter into the tube pan (or divide between the loaves) and top with 1/3 of the chocolate batter. Repeat, ending with the remaining chocolate batter on top.