Make your yellow cake layers:

1. Prepare 2 6-inch round cake pans with baking spray and parchment paper. Preheat the oven to 325 degrees F.
2. In a large mixing bowl attached to a stand mixer, cream together butter and granulated sugar for 3-5 minutes until light and fluffy. Add eggs and egg yolks one at a time, scraping down the bowl in between additions. Add vanilla extract. Continue mixing on medium-high for 5 more minutes until the mixture is lightened in color and shiny.
3. In a separate medium bowl, mix together cake flour, baking powder, baking soda, and salt. Reduce mixer speed to low. Alternate adding this dry ingredient mix with the buttermilk, beginning and ending with dry mix. Order will be 1/3 dry, 1/2 buttermilk, 1/3 dry, 1/2 buttermilk, 1/3 dry. Mix until just combined.
4. Spread cake batter equally between two pans. Bake at 325 for 30-35 minutes until a toothpick inserted comes out clean. 
5. Allow cake layers to cool completely in their pans. Cake layer can be stored, wrapped in plastic wrap, at room temp for up to 1 week.

Make your chocolate buttercream:

1. In a large mixing bowl fitted with the paddle attachment of a stand mixer, cream butter and cream cheese until smoothish on medium-high speed. Reduce speed to low and slowly add sifted cocoa powder and powdered sugar one half cup at a time.
2. Once the frosting is smooth and deep brown, increase mixer speed to medium and add vanilla and salt. Once incorporated increase mixer speed to medium-high and add heavy cream one tablespoon at a time. Continue mixing 3-5 minutes until the frosting becomes lightened in color, smooth, and fluffy.

Assemble your cake.

1. Place on cake layer on a cake round or stand. Spoon chocolate frosting on top of the layer and use a spatula to create a smooth, even layer. Place the other cake layer on top. Smooth chocolate frosting on the top and all around, using a bench scraper and/or offset spatula. Decorate with sprinkles to your preference.