Preheat over to 350 degrees.
Heavily coat 2- 9″ round baking pans with a non-stick spray like Baker’s Joy. Set aside.
Combine flour, cake flour, baking powder, baking soda & salt in a mixing bowl & whisk to sift. Set aside.
In the bowl of an electric mixer, whip together butter & granulated sugar using the paddle attachment.
Combine eggs & yolks into a measuring cup.
Pour in eggs, oil & vanilla into butter mixture & continue to beat.
Add in 1/3 of the flour mixture & mix until just combined.
Pour in 1/2 the buttermilk, mix.
Repeat the process again with the remaining flour & buttermilk, scraping down the sides of the bowl between mixing.
Divide the batter between the prepared cake pans.
Bake 25-30 minutes or until toothpick test is clean.
Cool in pans at least 10 minutes.
Run a knife around the edges to loosen from the pan.
Invert the cakes onto a wire rack & continue to cool.
After about 20 minutes of cooling, wrap each cake layer in plastic wrap to help the cakes retain their moisture. (especially if you live in a dry climate)
Chill the cakes in the refrigerator or freezer prior to decorating to make spreading the frosting easier.
Frost with chocolate buttercream frosting.
Store in an airtight container.