Preheat the oven to 350 degrees F. Grease and flour 2 8-inch cake pans.
In a medium bowl, whisk together the flour, baking powder and salt.
In a stand mixer fitted with the paddle attachment, cream together the butter, oil and sugar on medium speed until light and fluffy, about 3-4 minutes.
To the stand mixer bowl, add the eggs, egg yolk, vanilla and orange zest. Beat until well combined, scraping down the sides of the bowl as needed.
In a measuring cup, combine the milk and orange juice.
With the mixer on low speed, alternate adding the flour mixture and milk/OJ mixture, in 3 instalments each, beating until JUST combined (do not over beat).
Remove the bowl from the stand mixer and give it a good stir with a rubber spatula, scraping the bottom and sides of the bowl. Divide the batter evenly between the prepared cake pans and bake for 30-35 minutes until a toothpick inserted in the center comes out clean.
Allow the layers to cool in the pan for 15-20 minutes and turn them over onto a cooling rack to cool completely.