Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.

Cream the unsalted butter and oil in a large mixing bowl until smooth and well combined.

Add the granulated sugar gradually, beating until the mixture is light and fluffy.

Incorporate the eggs and egg yolks one at a time, mixing well after each addition. Stir in the vanilla extract.

Whisk together the baking powder, baking soda, salt, and flour in a separate bowl.

Combine the dry ingredients and buttermilk into the butter mixture, alternating between the two.

Divide the batter evenly between the prepared cake pans and smooth the tops with a spatula.

Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean.

Cool the cakes in the pans for 10 minutes before transferring to wire racks.

Prepare the chocolate frosting by combining melted butter, cocoa powder, powdered sugar, milk, and vanilla extract.

Frost the top of one cooled cake layer, then gently place the second layer on top. Spread frosting on the top and sides.