Soft dark chocolate cake layers, creamy sprinkle vanilla buttercream, and mint green water ganache drip.
Preheat the oven to 350 °F. Spray three 6-inch round cake pans or two 8-inch round cake pans with non-stick spray, line the bottom of each pan with parchment paper, and spray again. Set aside.
In the stand mixer bowl fitted with a whisk attachment, stir together the all-purpose flour, cocoa powder, granulated sugar, baking soda, and salt. Set aside.
Add the eggs and the vegetable oil on low until well incorporated.
Pour in the buttermilk and add brewed coffee. Mix on medium for 30 seconds. Scrape down the sides of the bowl.
Evenly distribute the cake batter among the three cake pans and bake for about 35-40 minutes or until a toothpick inserted into the center of each cake comes out clean or with just a few moist crumbs.
Remove cakes from the oven and let cool in pans for 10 minutes before inverting onto cooling racks to cool completely — wrap cakes in plastic wrap and place in the freezer to chill for 30 minutes.
In the stand mixer bowl fitted with the paddle attachment, beat the butter until light and fluffy, about 2 minutes.
Turn the mixer to low and add the salt and vanilla.
Next, gradually add powdered sugar, followed by the cream. Mix on medium-high speed for about 5 minutes, until light and fluffy. Scrape down the sides of the bowl.
Receive 1 cup of white (no sprinkle) buttercream for piping.
Add rainbow sprinkles to the remaining buttercream and mix by hand with a rubber spatula. (Note: Do Not use non-parels as those will bleed into the buttercream turning it into different colors).
Before decorating, use a rubber spatula or wooden spoon to push out the air bubbles. (You may need to repeat this step when frosting the outside of the cake).
Add white candy metals in a medium glass bowl with 1-2 yellow pieces, a few blue pieces, and a small handful of green candy melts. This combination will create a mint green color.
Melt mint green candy melts in the microwave. Heat for 30 seconds at a time until the chocolate candy melts have mostly melted.
Then mix in the warm water with a rubber spatula. Continue to mix until the chocolate and water are well combined and streak-free.
If the candy melts look oily or broken, add 1 TBSP of warm water at a time until the mixture is smooth. (Note: If there are chunks of chocolate, melt for 10 seconds at a time until the chocolate is fully melted or pour the chocolate into a fine mesh sifter to remove any chocolate lumps).
When ready to use, pour the chocolate into a squeeze bottle for a comfortable grip for decorating. If the water ganache drip becomes too cold and hardens, then heat in the microwave for 5-10 seconds, stir to make sure it's all the same temperature. (Note: Don't overheat, or chocolate will separate, and you'll have to remix, you may need to add more candy melts or warm water).
Remove the cake layers from the freezer and unwrap the cake layers. Next, use a serrated knife to cut off the dome of the cakes to make the layers flat and even.
Add a small amount of buttercream onto the center of the cake board. Layer one dark chocolate cake, cut side up onto the cake board. Add a 2/3 cup of sprinkle buttercream over the cake using an offset spatula to spread buttercream and make it even.
Place another cake layer cut side down over the top.
Repeat the process for another two layers — Crumb coat with sprinkle buttercream and freeze for 30 minutes.
Add a thicker layer of sprinkle buttercream to the cold cake. Use an offset spatula and cake scraper to make the sides as smooth as possible.
Place in the freezer to chill for 10-15 minutes.
Squeeze and drop the mint green water ganache, drip over the sides and on top. Use the offset spatula to smooth the top of the cake.
Fill a piping bag ⅔ of the way with white buttercream that's fitted with a #865-french piping tip—pipe dollops on the top of the cake.
Serve the cake at room temperature and enjoy! Use a hot knife to cut and get clean slices.