For the cake:

• Position a rack in the middle of the oven and preheat to 350°F (180°C). Grease a large 10-cup Bundt pan generously with butter or nonstick cooking spray, making sure to coat every crevice. Then, dust the entire surface with flour, shaking off any excess.
• In a medium bowl, sift together the all-purchase flour, potato flour, baking powder, and salt. Whisk to combine and set aside.
• In the bowl of an electric mixer fitted with a whisk attachment, beat the softened butter until pale and creamy, about 2 minutes. Add the granulated sugar and beat on high until the mixture is light and fluffy, about 3 to 5 minutes. Pause to scrape down the sides and bottom of the bowl as necessary.

  Add the eggs one at a time, beating well after each addition. Scrape down the sides and bottom of the bowl again to ensure the batter is evenly mixed. Add the oil and vanilla extract, and beat the mixture on high for 2 minutes.
• Gently fold in the flour mixture using a rubber spatula. Add the flour mixture in three additions, mixing until just combined. Do not overmix, as this can make the cake too dense (but ensure everything is evenly incorporated). Scrape down the bowl.
• Divide the mixture into two equal portions, placing each in a separate bowl.
• COCOA BATTER: Add the sifted cocoa powder to one of the mixtures and mix gently until smooth, creating a brown batter.
• ORANGE BATTER: Add the orange zest to the bowl with the remaining batter and mix well until smooth and golden.
• Using two large spoons, scoop the chocolate and orange batter into the prepared Bundt pan in large dollops, alternating the flavors as you go. Using an offset spatula or butter knife, gently smooth the surface.
• Bake for about 45-50 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool in the pan for 15 minutes before removing it and transferring it to a serving platter.

  Dust the top with powdered sugar before serving and slice with a serrated knife.