Step 1

Preheat the oven to 325°F / 162°C. Line a 9x13-inch (quarter sheet) pan with parchment paper. Place two flower nails / heating cores in the center of the pan with the flat side sitting against the cake pan. Make sure they are a few inches apart. Spray the pan and flower nails with nonstick baking spray.

Step 2

Place 1 cup of butter and 1 1/2 cups of sugar in the bowl of a stand mixer or a large bowl. Beat on a medium-high speed using a whisk attachment for 2 minutes. The mixture should become lighter in color as air is incorporated into it.

Step 3

Mix in 4 eggs on a low speed, two at a time. Scrape down the sides and bottom of the bowl as needed with a rubber spatula.

Step 4

In a separate bowl, whisk together the 3 cups cake flour, 1 Tbsp baking powder, and 1 tsp salt.

Step 5

Add half the dry ingredients into the butter/egg mixture and mix on a low speed until combined.

Step 6

Then mix in 1 cup buttermilk, 1/3 cup oil, and 1 Tbsp vanilla extract on a low speed.

Step 7

Add in the rest of the dry ingredients and mix until the ingredients are just incorporated. Scrape down the sides and bottom of the bowl as needed with a rubber spatula.

Step 8

Pour the batter into the prepared pan. Reposition the flower nails so that they’re evenly spaced across the pan.

Step 9

Bang the pan on your counter to remove any air bubbles, then bake for 35-40 minutes.

Step 10

Rotate the pan halfway through to help the cake bake evenly.

Step 11

Let the cake cool in the pan for about 15 minutes, then carefully remove it from the pan and place it on a wire rack to finish cooling. Once it's fully cooled, carefully flip the cake and remove the heating cores.

Step 12

While the yellow sheet cake bakes and cools, make the chocolate buttercream frosting.

Step 13

Melt 2/3 cup of chocolate first so that it has time to cool off before it's added into the frosting. Heat it in 15 second intervals, stirring between each interval until the chocolate is fully melted. Set aside.

Step 14

Beat 1 cup of butter on a medium speed for 30 seconds in a stand mixer with a paddle attachment or in a large bowl with a hand mixer until smooth.

Step 15

Mix in 1/2 cup cocoa powder and 1/2 tsp fine on a low speed until combined.

Step 16

Gradually mix in 3 cups of powdered sugar and 1/3 cup of heavy cream on a low speed. Scrape the sides and bottom of the bowl as needed.

Step 17

Then mix in the cooled, melted chocolate on a low speed.

Step 18

Beat on low until the ingredients are fully incorporated and the desired consistency is reached.

Step 19

If the frosting seems too thick, add an extra Tbsp of heavy cream. If it's too thin, add in another 1/4 cup of powdered sugar.

Step 20

Stir by hand with a rubber spatula for a couple minutes to get the frosting SUPER smooth.

Step 21

Spread an even layer of chocolate buttercream on top of the cooled sheet cake. Make swirls with an offset spatula or the back of a spoon.

Step 22

Add rainbow sprinkles around the perimeter of the cake and use a sharp knife to cut the cake into 16 squares. Then enjoy!