2 metal icing flower nails, an icing comb, piping bag and open-star cake decorating tip, such as Wilton 1M
For the yellow cake: Preheat the oven to 350 degrees F. Line a 9-inch-by-13-inch (quarter-sheet) pan with parchment paper, then spray with nonstick baking spray.
Place 2 metal flower nails in the center of the pan, flat-side down, a few inches apart. Beat the butter and granulated sugar in the bowl of a stand mixer fitted with the whisk attachment on medium-high speed to incorporate air into mixture, about 2 minutes. Mix in the eggs on low speed, two at a time. Scrape down the sides of the bowl as needed with a rubber spatula.
Whisk together the cake flour, baking powder and salt in a separate bowl. Add a third of the dry ingredients to the butter mixture and mix on low speed. Mix in half of the buttermilk on low speed. Repeat this process, finishing with the last third of the dry ingredients. Mix in the vanilla extract and oil, then pour the batter into the prepared pan. Reposition the flower nails so that they're evenly spaced in the pan. Tap the pan on the counter to remove any air bubbles, then bake until a toothpick inserted in the center comes out clean or with a few moist crumbs, 25 to 26 minutes.
Allow the cake to cool in the pan for about 15 minutes, then carefully flip it onto a cooling rack to finish cooling.
For the dark chocolate buttercream: Beat the butter in a stand mixer fitted with the paddle attachment on medium speed until smooth, about 30 seconds. Turn the mixer to low speed and add the cocoa, then mix until the cocoa is fully incorporated and no clumps remain. Slowly add the powdered sugar, 1 cup at a time, alternating with small splashes of cream. Mix in the melted dark chocolate, then the salt. Beat until the ingredients are fully incorporated and the desired consistency is reached. If the frosting is too thick, add an extra tablespoon of heavy cream. If it's too thin, add another 1/2 cup powdered sugar. Stir by hand with a rubber spatula for a couple minutes to make the frosting super smooth.
Remove the flower nails from the cake. Spread an even layer of chocolate buttercream on top of the cake, making swirls with an offset spatula. Top with rainbow sprinkles.