Preheat oven to 350°F and line the bottoms of three 6” cake pans with a round of parchment paper. Set aside.
In a medium bowl, stir together flour, cocoa powder, baking soda, baking powder, and salt. Set aside.
In a large mixing bowl, whisk together melted butter, oil, granulated sugar, and brown sugar, and eggs until combined.
Add half of the flour mixture to the butter mixture and whisk together until just combined.
In a measuring cup, add in the orange juice, orange zest, and milk. Pour half of this orange juice mixture into the batter and whisk until just combined.
Add in the remaining flour mixture and then the orange juice mixture, whisking in between additions until just combined and the batter is smooth and glossy.
Divide the batter into your three lined cake pans until about ⅓ full (I portioned out about 360g of batter into each pan).
Bake cakes for 35-40 minutes or until a toothpick inserted in the middle comes out clean.
Let cakes cool in the pans for 10-15 minutes or until cool enough to handle. Run a knife around the edges of the cake pan. Invert the cake pans to release the cake layers onto a wire rack or parchment paper and let them cool completely while upside down before assembling cake.
Buttercream
In the mixing bowl of a stand mixer, use the paddle attachment to beat softened butter on medium-high speed for 10 full minutes until it becomes very creamy and pale in colour.
Add in the powdered sugar and beat on medium-high speed for another 10 minutes, scraping down the bowl when needed. If you’d like to colour your buttercream for decorating, remove a few spoons of buttercream at this point and transfer them to small bowls. Add gel food colouring and stir to mix thoroughly. Set aside.
To the rest of the buttercream, add in the melted dark chocolate. Beat again for 5 minutes until thoroughly mixed. Finish by smearing the buttercream around the bowl with a spatula to get rid of air bubbles.
Assembling the cake
Lay down one cake layer on a turntable. Spread an even layer of buttercream on top with an offset spatula. Place the second cake layer on top. Repeat with all your cake layers.
Spread a thin crumb coat of buttercream on the top and sides of the cake and use a bench scraper to remove the excess. Chill in the fridge for 10 minutes until firm.
Spread a final layer of buttercream on the cake and smooth with a bench scraper.
Transfer the remaining buttercream and coloured buttercream to piping bags fitted with piping tips of your choice. Pipe your design on your cake.