To make the cake:

1. Preheat the oven to 350 degrees and grease three 8 inch cake pans. Line the bottoms of the pans with parchment paper and set aside. 
2. Sift the flour, baking soda, baking powder and salt together in a small bowl and whisk a few times to combine. Set aside.
3. Add the butter to a large mixing bowl (I used a 6 quart size) and beat with a hand mixer on medium high speed until creamy, about one minute.
4. Add the granulated sugar and brown sugar and beat on medium high for another minute. The butter and sugar won’t form a paste, it will look like very wet sand.
5. Add the oil and beat for another minute, until the mixture lightens in color and is slightly fluffy.
6. Add the eggs and yolks, one at a time, beating well after adding each one.
7. Add the vanilla extract and yellow food coloring and mix just until blended.
8. Add half of the buttermilk and half of the flour mixture and mix on low speed JUST until there are only a few flour streaks remaining. Add the remaining buttermilk and flour and mix again on low speed. Fold the last few pockets of flour into the batter with a spatula. Do not overmix the batter.
9. Divide the batter evenly among the three prepared cake pans, and smooth the surface with a butter knife or small offset spatula. 
10. Bake the cakes for 21-23 minutes, rotating the pans halfway through if your oven has hot spots. The cakes should spring back just barely if you gently press the center with your finger.
11. Remove the cakes from the oven and allow them to cool for 5-6 minutes. The pans will still be too hot to handle with bare hands, so carefully invert each cake layer onto a sheet of plastic wrap and wrap each layer securely, being sure to seal the edges/sides. 
12. Load the cakes into the freezer and allow them to chill for 2 hours, or until they are fully cold and the edges are beginning to freeze. At this point, if you’re serving the cake the next day, keep the layers wrapped and in the fridge until you’re ready to use them. 

To make the buttercream and finish the cake:

1. Add the butter to the bowl of a stand mixer and use the paddle attachment on medium high speed to beat until smooth and creamy, about a minute. Scrape down the bowl and add the remaining ingredients.
2. Mix on low speed until the powdered sugar and cocoa have mostly been incorporated. Increase the speed to medium high again and beat for 2-3 minutes, until the frosting is smooth, lighter in color and fluffy.
3. Unwrap the first cake layer and use a serrated knife to carefully level off the top of the cake. You may not need to do this if your cakes flattened out after being removed from the oven.
4. Add the leveled cake layer to a large plate or decorating turntable (preferred). Add about 2/3 cup of the chocolate buttercream to the top of the cake and use an offset spatula to cover the top in a smooth, even layer.
5. Repeat the process with the second and third cake layers, stacking them on top of each other with the buttercream in between. Use the remaining chocolate frosting to cover the sides of the cake. You can smooth the outside of the cake with a bench or cake scraper.
6. Decorate the cake with sprinkles, if desired, and serve! Makes 8 large slices, and up to 12 medium/small slices.