• For the cake: 

1. Preheat the oven to 350°F. Prepare three 8-inch round baking pans, line the bottom with parchment paper, and grease the sides. 
2. In a large mixing bowl, combine the sugar, vegetable oil, eggs, egg yolks and vanilla extract. Beat on medium speed until the eggs and oil are well incorporated. Next mix in the sour cream and beat until well combined.
3. In a separate bowl, combine and sift the remaining dry ingredients. Alternate adding half the dry ingredients and half the buttermilk at a time and mix at a low speed just until the flour starts to incorporate. Then add the remaining dry ingredients and buttermilk, beating until well combined. Use a spatula to scrape down the sides of the bowl and stir the batter from the bottom to the top to ensure it’s well mixed.
4. Divide the batter evenly between the three pans, about 1 2/3 cups of batter per pan. Bake at 350°F for 23-25 minutes. Test the cake for doneness by inserting a toothpick into the center of the cake. If the toothpick comes out clean, your cake is done. Cool completely. Remove from the oven and allow to cool completely.

For the frosting: 

1. Cut the butter into pieces (about 1 tablespoon in size). Using the paddle attachment, whip  the butter at a high speed for 5-7 minutes, scraping down the bowl occasionally. Beat until the butter is light, fluffy and easily spreadable. Next, beat the cocoa powder into the butter until well-mixed.
2. Alternate adding 2 cups of powdered sugar at a time with the liquid ingredients: vanilla extract & heavy cream. Ensure the powdered sugar is well combined, and whip for 1-2 minutes before adding more powdered sugar. Finally, add the pinch of salt, increase the speed to medium-high and beat for another 2-3 minutes to whip additional air into the frosting.

To assemble:

1. Level the cake layers by removing any domes. Place a dollop of frosting on a cake board and place the bottom layer of cake over that.
2. Using a large piping bag that is half filled, pipe the first layer of frosting. Pipe in circles working outside edge to the middle. Spread evenly with an offset spatula. Add the second layer of cake and repeat. Place the third layer of cake on top.
3. Use an offset spatula to press any excess frosting onto the edges of the cake. To finish, pipe the frosting along the outside edge of the cake using a large open round tip or a cake icer tip. Then use an offset spatula or icing smoother to create a smooth the edges.
4. Decorate the top with a 1M piping tip and sprinkles. You’ll need about 2 cups of frosting for the border on top of the cake. To press sprinkles into the sides of the cake, do this right away, and place a large baking sheet under the cake to catch extra sprinkles.