Line two 8 or 9 inch round cake pans with parchment paper and spray pans with nonstick cooking spray.
In a large bowl mix sugar and butter for a 3 minutes. Add oil and mix well 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.
Chocolate Frosting:
Melt butter and stir in cocoa powder. Mix in the powdered sugar, milk ,and vanilla and mix for a few minutes, until frosting is light and fluffy. Once the cake has cooled completely, frost with chocolate frosting.
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Notes
To Make Ahead: Store chocolate frosting in an air-tight container in the fridge for up to two weeks. Bring to room temperature before using.To Freeze: The cakes and frosting can be frozen. Allow the cakes to cool completely, then wrap in plastic wrap and store in a freezer bag for up to one month. Frost cakes when they are still frozen. Freeze frosting in an airtight container for up to three months.For 9x13 Pan Yellow Cake: Bake for about 30-35 minutes or until a toothpick inserted in the center comes out clean.For YellowCupcakes: Bake for about 17-22 minutes, or until a toothpick inserted in the center comes out clean.