Preheat the oven to 325 degrees. Prepare three 8-inch or four 6-inch round cake pans by lightly spraying the bottom of the pan with cooking spray. Line with parchment paper and spray again. Set aside.
In a medium sized bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter, oil, and sugar on high until fluffy and smooth, about 4 minutes. With a rubber spatula, scrape the bottom and sides of the bowl.
Add the whole eggs, and egg yolks one at a time, beating on medium speed after each addition. I usually beat each egg for about 30 seconds. Scrape the bowl after each addition. Once all the eggs/egg yolks are added, beat for 30 more seconds.
Add the sour cream, butter extract, and vanilla. Beat on medium low speed for a minute. If the mixture looks clumpy, don’t worry, once you add the flour mixture it will smooth out.
Turn the mixer on low and add half of the flour and ½ of the buttermilk to the butter mixture and mix ON LOW until almost incorporated. Add the rest of flour and the rest of the buttermilk. Mix just until a few streaks of flour remain. With a rubber spatula, gently mix the bottom and the sides of the mixture!
Evenly distribute the cake batter among the three 8-inch or four 6-inch pans and bake for 28-32 minutes or until a toothpick comes out with just a few moist crumbs on it or until it starts to get a little golden brown on top.
Remove cakes from the oven and let them cool in the pans for 15 minutes before transferring them out of the pan and placing them on cooling racks.
When cakes are completely cooled, wrap each cake layer individually with plastic wrap and stick it in the refrigerator or freezer until you’re ready to frost.
For the chocolate buttercream
In a microwave safe bowl, heat the chocolate for 30 seconds, stir, heat for 30 more seconds, and set aside. The chocolate will not be melted all the way, but this will help it melt quicker when you add the cream.
In another microwave proof bowl, heat the cream until nearly boiling. Make sure it doesn’t boil over. Pour the hot cream over the slightly warm chocolate chips. Let it sit for 2 minutes and then stir. Set aside and let it cool.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter for about 2 minutes on medium-high speed.
Add the chocolate/cream mixture. Scrape down the sides and bottom of the bowl with a rubber spatula and beat for 1 more minute.
Add the powdered sugar, vanilla, and salt. Mix on low until combined, then turn the mixer to medium high and beat for 3 minutes.
With a wooden spoon or rubber spatula, beat by hand to mix out any air pockets.
Assembly
Place the first cake layer, top side up. Add about 1 cup of frosting. Use an off set spatula to spread over the cake layer as evenly as possible to the edges of the cake.
Repeat this step until you get to the final cake layer, which you will lay top side down.
Apply a thin coat of buttercream around the entire cake to lock in the crumbs. Freeze for about 10 minutes.
After the crumb coat is set, apply the rest of the buttercream. I used Wiltons 8B to obtain the shells.
This cake is served best at room temperature. Wrap the cake in plastic wrap and it can be stored in the fridge for up to 5 days. You can also wrap each individual slice in plastic wrap and store in the freezer for up to 30 days.