Preheat your oven to 350°F and grease and flour two 8-inch cake pans.

In a medium bowl, whisk together the flour, baking powder, and salt.

In a large bowl, beat together the butter, oil, and sugar on medium speed for 3-4 minutes, or until light and fluffy.

Add the eggs, egg yolk, vanilla, and orange zest, and beat until fully incorporated, scraping down the sides of the bowl as needed.

In a separate measuring cup, combine the milk and orange juice.

With the mixer on low, gradually add the dry ingredients and milk/juice mixture in three alternating batches, beginning and ending with the flour mixture. Mix just until combined—avoid overmixing.

Divide the batter evenly between the prepared pans and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.

Let the cakes cool in the pans for 15-20 minutes before turning them onto a cooling rack to cool completely.

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

In a large bowl, beat the butter and cream cheese on medium speed until smooth. Then, add the vanilla extract and orange zest, and mix until combined.

Gradually add the icing sugar mixture, one cup at a time, beating after each addition until the frosting is smooth and creamy.

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