Preheat the oven to 325°F and grease and flour two 8-inch cake pans. Set aside.

In a large mixing bowl fitted with a whisk attachment, beat the softened butter and oil together until they become homogenous, about 3 minutes.

Slowly add the granulated sugar to the oil and butter, beating on high until the mixture is light and fluffy and turns a very pale yellow, about 3 minutes.

Add the eggs and egg yolks in one at time, scraping down the bowl in between each addition. Add the vanilla extract. Beat the mixture on high for 3 minutes more.

In separate bowl sift together the flour, salt, and baking powder. Alternate mixing in the dry ingredients and the buttermilk in 4 batches, starting with the buttermilk and ending with the dry ingredients.

Split the batter evenly into the prepared pans and bake for 30 minutes or until the cake is a lightly golden brown and springs back when pressed.

Place the butter in the bowl of a stand mixer fitted with the paddle attachment. Beat the butter for 30 seconds to soften it up.

Add the cocoa powder, powdered sugar, milk, corn syrup, salt, and vanilla extract. Beat on low speed gradually increasing to high until glossy and smooth, about 1 minute.

Place the first cake on a cake stand, then top with about a third of the frosting. Top with the second cake and top with another third of the frosting. Frost the edges of cake with remaining frosting.