Preheat oven to 350 degrees F.

Line two 8 inch round cake pans with parchment paper and spray pans with nonstick cooking spray.

In a large bowl mix sugar and butter for 3 minutes. Add oil and mix again. Add the eggs and yolks, one at a time, beating after each addition. Add vanilla and mix.

Stir together baking powder, baking soda, 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, stirring 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. Don't over-bake or the cake will be dry.

Invert cakes onto a wire cooling rack to cool completely.

Melt butter, then stir in cocoa powder. Mix in the powdered sugar, milk, and vanilla then mix for a few minutes, until frosting is light and fluffy. Once the cake has cooled completely, frost with chocolate frosting.