Preheat your oven to 350 degrees F (175 degrees C). Prepare two 9-inch round baking pans by buttering them well.

Line the bottoms with parchment paper. Butter the parchment paper as well. Lightly dust the pans with flour and shake out any excess flour. Set the prepared pans aside.

In a mixing bowl, sift together the all-purpose flour and cake flour. Add baking powder, baking soda, and salt. Whisk these ingredients together for about 20 seconds. Set this bowl aside.

In the bowl of your electric stand mixer fitted with the paddle attachment, add the softened butter and granulated sugar. Beat them together until very pale and fluffy.

Blend in the vegetable oil with the butter and sugar mixture until it's well combined.

Add the eggs one at a time, mixing well after each addition. Then, add the egg yolks and vanilla extract, mixing until well combined.

Add one-third of the flour mixture to the wet ingredients and mix just until combined.

Pour in half of the buttermilk and blend until mixed in well. Repeat this process: add another one-third of the flour, then the rest of the buttermilk, followed by the last of the flour mixture.

Scrape down the sides and bottom of the bowl with a rubber spatula to mix everything evenly.

Divide the batter evenly between the two prepared cake pans.

Bake in the preheated oven for 28 to 35 minutes. To check if the cakes are done, insert a toothpick in the center. If it comes out clean, the cakes are ready.

Allow the cakes to cool in the pans for 10 to 15 minutes. Run a knife around the edges to loosen them, then invert onto wire racks to cool completely.

Once the cakes are cool, frost them with the chocolate buttercream frosting.