Preheat oven to 350℉. Grease and flour two 9-inch round deep cake pans. Set to the side.

To make the cake, in a large bowl, combine the flour, baking powder, and salt. Set aside. In a separate large mixing bowl, cream together the butter and sugar on high for about 5 minutes or until light and fluffy. Add the eggs and egg yolks, one at a time, mixing well after each addition. Stir in the vanilla extract and scrape the bottom and sides of the bowl with a rubber spatula to ensure everything gets evenly combined. Add the dry ingredients and the buttermilk to the batter, alternating between the two. Begin and end with the dry ingredients. So add a bit of dry, mix until just combined, buttermilk, mix, dry, mix, milk, mix, and dry, until it's all incorporated. Divide the batter between the prepared cake pans. Give them a few raps on the counter to dislodge any air bubbles.

Bake for about 25 to 30 minutes or until golden brown and a toothpick inserted in the middle comes out clean. Remove from oven and allow to cool for about 10 minutes. Run a knife along the edge of the cake and then invert to remove the cake from the pan. Allow to cool completely on a wire rack.

To make the buttercream, in a large bowl, sift together the powdered sugar, cocoa and salt. Set aside. In a separate bowl, cream the butter on high for about 3 minutes until light and fluffy. Slowly add in the dry ingredients, mixing on low until combined. Once all added, raise the speed to high and mix until combined. Add the heavy cream or milk, 1 tablespoon at a time mixing until the frosting it light, fluffy and spreadable. Not too thick, but also not too thin.

Level off the cooled cakes by trimming the tops to even them out, using a serrated knife. Place one layer on a cake plate or stand and add a dollop of frosting. Smooth it out with an offset spatula into an even layer. Top with the second cake layer, upside down, so that you end up with a flat surface on top. Frost the top and sides of the cake and smooth out as best you can with the offset spatula. Decorate the edges with a piping bag if you wish, or douse the top with sprinkles. Keep at room temperature until ready to cut and serve.

If you wish to keep it longer than a day before cutting, store in the fridge, covered with plastic wrap. Store any leftovers in the fridge as well, wrapped in plastic. Enjoy!