Step 1

Generously coat a 12-cup Bundt pan with cooking spray.

Step 2

Combine sugar and orange zest in the bowl of a stand mixer fitted with the paddle attachment. Mix for a few seconds to release the oils from the zest. Add butter and oil and beat on medium speed until light and fluffy, about 1 minute. Add eggs, one at a time, scraping down the sides of the bowl between additions. Beat until smooth.

Step 3

Sift all-purpose flour, whole-wheat flour, baking powder, baking soda and salt into a large bowl. Add the flour mixture into the batter in 3 additions, alternating with 2 additions of buttermilk. Scrape the batter into the prepared pan (it should be no more than two-thirds full).

Step 4

Place the cake in a cold oven for 20 minutes. Then set oven temperature at 325°F and bake until the cake is golden brown, pulls away from the sides of the pan and a cake tester inserted in the center comes out clean, about 1 hour.

Step 5

Run a thin knife around the sides of the pan to release the cake. Transfer to a wire rack to cool in the pan for 10 minutes. Invert the cake onto the rack, remove the pan and let cool completely, about 1 hour.