To the buttermilk add the baking soda and set aside. Next, sift together the flour, baking powder, and salt; set aside as well.
Cream together the butter, sugar, canola oil, and vanilla extract, followed by the eggs and yolks one at a time.
Alternate adding the flour and milk. Start by adding one-third of the flour, half the buttermilk mixture, and repeat ending with the flour to the creamed butter.
Pour the batter into 3-8” round buttered and floured cake pans. Bake at 325° 24-28 minutes or until a toothpick comes out clean. Cool completely on baking racks before frosting.
Whip the melted butter, vanilla, salt, and cocoa powder until smooth. Add the chocolate mixture to a stand mixer and slowly beat in the powdered sugar alternating with the heavy cream. Once the powdered sugar is incorporated, whip on medium speed for 2-3 minutes until fluffy. Frost the cakes generously with the icing.
Do not over mix the batter. Turn your hand-mixer or stand-mixer on low speed when you begin add the flour and buttermilk.