This cake is soft and moist with the perfect combination of density and fluff to it, it's somewhere just between the two. And when you top with with a rich, melt-in-your-mouth chocolate buttercream frosting you know it's got to be good!
Preheat oven to 350 degrees F. Butter two 9-inch round baking pans, line with parchment paper and butter parchment.
Lightly dust pans with flour and shake out excess, set pans aside. For the most level cakes you can also wrap a soaked cake strip around each pan.
Sift all-purpose flour and cake flour into a mixing bowl. Add baking powder, baking soda and salt and whisk 20 seconds, set aside.
In the bowl of an electric stand mixer fitted with the paddle attachment whip together butter and granulated sugar until very pale and fluffy (if not using a paddle attachment that constantly scrapes bowl, then stop mixer occasionally throughout entire mixing process and scrape down sides and bottom of bowl).
Blend in vegetable oil. Mix in eggs one at time then blend in egg yolks and vanilla extract.
Add in 1/3 of the flour mixture then mix just until combined, then add in 1/2 of the buttermilk and blend until combined, repeat process once more then finish by mixing in remaining 1/3 of the flour mixture. Scrape down sides and bottom of bowl with a rubber spatula.
Divide batter among prepared cake pans. Bake in preheated oven 28 to 35 minutes until toothpick inserted into center comes out clean.
Cool in pans for 10 to 15 minutes. Run knife around edge to make sure they are loose, then invert onto wire racks to cool (once halfway cool I recommend wrapping with plastic wrap until completely cool).
Frost with chocolate buttercream once cool.
Let the yellow cake layers cool completely before topping them with chocolate frosting.
Store this cake in an airtight container to keep it from drying out.