Preheat and Prepare the Pans: Preheat your oven to 350°F (175°C). Grease two 9-inch round cake pans and line the bottoms with parchment paper for easy removal.
Mix the Dry Ingredients: Sift together flour, baking powder, baking soda, salt, and cocoa powder in a large bowl to ensure even distribution.
Cream the Butter and Sugar: In another bowl, beat softened butter, oil, and sugar with a hand or stand mixer on medium speed until light and fluffy, about 3 minutes.
Incorporate the Eggs and Flavorings: Add eggs one at a time, beating well after each addition. Mix in vanilla extract and orange zest to infuse the batter with citrus aroma.
Combine Wet and Dry Ingredients: Gradually add the dry ingredients and freshly squeezed orange juice to the butter mixture, alternating and starting and ending with the dry ingredients. Stir in milk to create a smooth batter.
Bake the Cake Layers: Divide the batter between the prepared pans, smooth the tops, and bake for 30-35 minutes, or until a toothpick comes out clean. Cool in pans for 10 minutes, then transfer to a rack to cool completely.
Prepare the Frosting: Beat softened butter until creamy. Gradually add powdered sugar and cocoa powder, mixing until combined. Stir in orange juice, zest, vanilla extract, and a pinch of salt until the frosting is light and fluffy.
Assemble the Cake: Once cooled, place one cake layer on a serving plate, spread frosting on top, then add the second layer. Frost the top and sides, and garnish with orange slices or zest if desired.