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 large bowl, cream together the butter, oil and sugar on medium speed until light and fluffy, about 3-4 minutes.

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).

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.

In a medium bowl, whisk together the icing sugar and cornstarch, breaking up any large lumps.

In a large bowl, beat together the butter and cream cheese on medium speed until smooth. Beat in the vanilla extract and orange zest.

Beat in the icing sugar mixture, one cup at a time, until the frosting is smooth and creamy.

Place one cake layer on a plate or turntable, and using an offset spatula, frost the top with about 1 cup of frosting. Place the second layer upside down on top and frost the top and sides of the cake. Decorate with orange slices if desired.