For the cake: 

1. Preheat the oven to 350°F. In a large mixing bowl, combine the sugar, vegetable oil, butter, eggs, egg yolks, and vanilla extract. Beat on medium speed until the eggs and oil are well incorporated and the batter is lighter in color, about 2 minutes. Scrape down the bowl as needed. Next mix in the sour cream and beat until well combined.
2. In a separate bowl, combine the remaining dry ingredients. Alternate adding half the dry ingredients and half the buttermilk at a time and mix just until the flour starts to incorporate. Then add the remaining dry ingredients and buttermilk and beat 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.
3. Pour the batter into a 9-inch by 13-inch pan and bake at 350°F for 25-30 minutes. Test the cake for doneness by inserting a toothpick into the center of the cake. If the toothpick comes out clean, the cake is done. Remove from the oven and allow to cool completely.

For the frosting:

1. Cut the butter into 1-inch blocks. Beat the butter for 3-4  minutes until it’s softened.
2. Then add the cream cheese and cream together for 2-3 minutes until they are fluffy and free of lumps, and scape down the bowl occasionally.
3. Add the cocoa powder and vanilla extract and beat until well combined,
4. Next, add the confectionary sugar 2 cups at a time, followed by one tablespoon of heavy whipping cream, and beat for 60-90 seconds until completely smooth. Add the remaining powdered sugar and cream and beat until desired consistency, at least 2 minutes.
5. Spread frosting over the cooled cake and then cover it with sprinkles.