Preheat your oven to 325 degrees F. Spray one 8" round cake pan with non-stick spray, line the bottom with parchment paper and spray again. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, combine the butter, oil, sugar and salt. Beat on medium speed for about 2 minutes, until light and fluffy.
Turn the mixer to medium-low and add the eggs and egg yolk, one at a time, scraping down the sides of the bowl in between each addition.
With the mixer on low, alternately add the flour and baking powder with the buttermilk, starting and ending with the flour. Mix until combined.
Add the vanilla extract. Mix on low until combined.
Bake the cake for about 25-30 minutes, until a toothpick inserted in the center of the cakes comes out with only just a few crumbs on it.
Cool the cake on wire racks for about 10 minutes. Carefully invert the cake onto cooking racks to cool completely. Once cooled, carefully divide the cake in half horizontally.
If you don't plan to stack and decorate the cake right away, level the cakes if needed and then immediately wrap each layer in plastic wrap or tin foil and freeze.
About an hour before assembly, remove the cake from the freezer.