step 1
Preheat oven to 350 degrees F (180 degrees C). Line a 9x13-inch (quarter sheet) pan with parchment paper. Place two flower nails in the center of the pan, with the flat side resting flat against the cake pan. Make sure they are a few inches apart. Spray with Non-Stick Baking Spray (as needed).
step 2
Place the Unsalted Butter (3/4 cup) and Granulated Sugar (1 1/2 cups) in the bowl of a stand mixer. Beat on medium-high speed using a whisk attachment for 2 minutes.
step 3
In a separate bowl, whisk together the Cake Flour (3 cups), Baking Powder (1 Tbsp), and Salt (1 tsp). Scrape down the sides of the bowl as needed with a rubber spatula to fully combine.
step 4
Mix in the Eggs (4) at low speed, two at a time.
step 5
Add ⅓ of the dry ingredients into the butter mixture and mix at low speed. Mix in half of the Buttermilk (1 cup) at a low speed. Repeat this process, finishing with the last 1/3 of the dry ingredients.
step 6
Mix in the Vanilla Extract (1 Tbsp), then pour batter into the prepared pan. Reposition the flower nails so that they’re evenly spaced across the pan. Tap the pan on your counter to remove any air bubbles, then bake for 25-26 minutes in the preheated oven.
step 7
Allow the cake to cool in the pan for about 15 minutes, then carefully flip it onto a cooling rack to finish cooling.
step 8
To make the Chocolate Buttercream Frosting, beat the Unsalted Butter (1 cup) on medium speed until smooth, about 30 seconds.
step 9
Add in the Unsweetened Cocoa Powder (1/2 cup), and mix on low until the cocoa is fully incorporated, and no clumps remain.
step 10
Slowly add in the Powdered Confectioners Sugar (3 cups), 1 cup at a time. Alternate with small splashes of Heavy Cream (1/4 cup).
step 11
Mix in the cooled, Dark Chocolate (2/3 cup).
step 12
Once fully mixed, add in the Salt (1/4 tsp). Beat on low until the ingredients are fully incorporated, and the desired consistency is reached. If the frosting is too thick, add an extra Tbsp of heavy cream. If it's too thin, add in another 1/2 cup of powdered sugar.
step 13
Allow the cake to cool completely, then remove the flower nails from the cake.
step 14
To assemble, spread an even layer of chocolate buttercream on top of the cake, making swirls with an offset spatula. Top with rainbow sprinkles if desired.
