Preheat the oven to 350°F (175°C). Grease and line the bottom of two 9-inch round cake pans with parchment paper.
In a mixing bowl, whisk together the flour, baking powder, baking soda, salt, and cocoa powder. Set aside.
In another large mixing bowl, combine the granulated sugar, brown sugar, buttermilk, vegetable oil, eggs, vanilla extract, orange zest, and orange juice. Beat with an electric mixer on medium speed until well blended.
Gradually add the dry ingredients to the wet mixture and mix until just combined. Be careful not to overmix.
Divide the batter evenly between the prepared cake pans. Smooth the tops with a spatula.
Bake in the preheated oven for 25 to 30 minutes, or until a toothpick inserted in the center comes out clean.
Once baked, remove the cakes from the oven and let them cool in the pans for 10 minutes. Then, transfer the cakes to a cooling rack to cool completely.
For the frosting, beat the softened butter in a mixing bowl until creamy. Gradually incorporate the powdered sugar and cocoa powder, alternating with the orange juice until you achieve a smooth consistency. Mix in the orange zest.
Once the cakes are completely cooled, spread frosting on top of one layer, place the second layer on top, and frost the top and sides of the cake.
Slice and serve. Enjoy your delicious Chocolate Orange Cake!