Place parchment paper in the bottom of two 8 or 9 inch round cake pans. Cut the paper so it fits well in the bottom circle of the pan. Spray the pan and parchment paper with nonstick cooking spray.
In a large bowl, cream sugar and butter until smooth. Add oil and mix
Add the eggs and yolks, one at a time, beating after each addition. Add vanilla and mix.
Stir together baking powder, salt, and flour in a medium bowl.
Alternate adding in some of the flour mixture, stirring to combine and then some of the buttermilk, ending with the flour mixture until just combined. Divide the batter evenly in the pans.
Bake for 25-35 minutes or until a toothpick inserted into the center of the cake comes out with few moist crumbs, but not wet batter. Don't over-bake or the cake will be dry.
Allow the cake to cool for a few minutes before inverting onto a wire cooling rack to cool completely.
For the Chocolate Frosting:
Stir together the butter and cocoa powder.
Use electric mixers or a stand mixer to beat in the powdered sugar, milk ,and vanilla until light and fluffy. I beat the frosting for several minutes.
Once the cake has cooled completely, frost with chocolate frosting.
Tips for making ahead and freezing the cake are above in the post.
*You can use salted butter, just reduce the amount of salt added to the batter.**You can substitute all butter if you want, instead of the oil, but I found the oil and butter combination yields a moister cake.This cake recipe is adapted from The Domestic Rebel