Preheat oven to 350 degrees. Grease and flour a 9-inch springform pan then set aside.
In a large bowl, sift together flour, cocoa powder, baking powder, baking soda and salt. Set aside.
In a separate mixing bowl, beat eggs, both sugars and orange zest with an electric mixer for 4 minutes until light and fluffy, scraping down the sides of the bowl with a spatula as needed.
Reduce mixer speed to low then gradually drizzle in olive oil. Beat until oil is evenly distributed.
Keeping mixer on low, add 1 cup of the flour mixture then beat until blended.
Next, add orange juice and buttermilk, beating until incorporated.
Finally, add remaining flour mixture and beat until just combined. Do not overmix. Note: batter will be thin.
Pour batter into prepared springform pan then bake for 55 to 60 minutes. Cake is done when a toothpick inserted into the center comes out clean and the top is golden brown. Avoid overbaking.
Place pan on a wire rack and allow cake to cool for 30 minutes before releasing the springform ring then allow cake to cool completely before frosting.