1. To make the cake, preheat oven to 350°F (176°C) and prepare three 8 inch cake pans with parchment paper in the bottom and baking spray on the sides.
2. Combine the buttermilk, eggs, egg yolks and vanilla extract in a large bowl, then separate about 3/4 cup of the mixture into another bowl or measuring cup. About 1 1/4 cups should remain in the other bowl. Set both aside.
3. In a large mixer bowl, combine the flour, sugar, baking powder, baking soda and salt.
4. With the mixer on the lowest speed, add the butter about a tablespoon at a time, allowing it to incorporate before adding the next tablespoon. As you add more butter, the mixture will start to clump together a bit a should end up resembling wet sand.
5. Add the larger of the reserved egg mixture (about 1 1/4 cups) to the dry ingredients/butter mixture. Stir on the lowest speed until it’s incorporated, then scrape down the sides of the bowl.
6. Increase the speed to medium high and beat until light and fluffy, about 45 seconds to a minute. Scrape down the sides of the bowl.
7. Turn the speed down to low and slowly add the remaining egg mixture in a slow stream until incorporated.
8. Scrape the sides of the bowl, then turn speed back up and mix until well combined, about 10-15 seconds.
9. Divide the batter evenly between the three cake pans and bake 20-25 minutes or until a toothpick inserted comes out clean.
10. Remove cakes from the oven and allow to cool for 2-3 minutes, then turn out onto a cooling rack to cool completely.
11. To make the frosting, add the butter to a large mixer bowl and beat until smooth.
12. Slowly add half of the powdered sugar and mix until smooth.
13. Add the cocoa powder and vanilla extract and mix until well combined and smooth.
Add 3-4 tablespoons of water or milk and mix until well combined and smooth.
14. Slowly add the remaining powdered sugar and salt and mix until smooth. Add additional water or milk, as needed to get the right consistency of frosting.
15. To put the cake together, use a large serrated knife to remove the domes from the top of the cakes so that they’re flat. Refer to my tutorial, if needed.
16. Place the first cake on a serving plate or a cardboard cake round.
17. Spread about 1 cup of frosting evenly on top of the cake.
18. Add the second layer of cake and another cup of frosting.
19. Top the cake with the remaining layer and frost the outside of the cake. Refer to my tutorial for frosting a smooth cake, if needed.
20. Use a decorating comb to add pattern to the sides of the cake, if desired.
21. Pipe shells of frosting around the top edge of the cake. I used Ateco tip 844.
22. Add a few sprinkles to the top of the cake. I used this sprinkle mix. Store in an air-tight container until ready to serve. Cake is best for 3-4 days.