Preheat the oven to 350°F. Lightly spray an 8x4 inch loaf pan with non-stick spray and set aside.
In a large bowl, whisk together the flour, baking powder, salt and orange zest. In another bowl, whisk together the brown sugar, egg, oil, buttermilk, orange juice and vanilla until well combined. Add this mixture to the dry ingredients and stir until just combined.
Move half of the batter to another bowl and whisk in the cocoa powder. To the other half of the batter, fold in the chocolate chips.
Fill the prepared loaf pan with alternating spoonfuls of the two batters. Swirl a knife through the batter to mix the two together slightly and smooth the top. Bake the loaf for 45-50 minutes, or until a toothpick inserted in the center comes out clean. Allow to cool for 10 minutes in the loaf pan, and then turn out onto a wire rack to cool completely.