Soft layers of dark chocolate cake filled with nutella buttercream and marshmallow fluff.
Preheat 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 paddle attachment, stir together the all-purpose flour, granulated sugar, dark cocoa powder, 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 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 and nutella until light and fluffy, about 2 minutes.
Turn the mixer to low and add cocoa powder. Then gradually add the powdered sugar, followed by the salt, vanilla extract and heavy cream. Mix until combined. Mix until incorporated completely. Scrape down sides of the bowl.
Turn the mixer to medium-high and beat the frosting for about 5 minutes until light and fluffy. Add a tiny dot of green and yellow gel food color as needed. Then turn the mixer on low and mix for 2 minutes to push out any air bubbles.
In a medium glass bowl, melt white chocolate candy melts and water in the microwave. Heat for 30 seconds at a time until the chocolate candy melts have mostly melted.
Then mix with a rubber spatula. Continue to mix until the chocolate and water are well combined and streak free. 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.
Pour the chocolate into a squeeze bottle for a comfortable grip for decorating. If chocolate drip becomes too cold and hardens then heat in the microwave for 5-10 seconds, shake or stir to make sure its all the same temperature. Don’t overheat or chocolate will separate, and you’ll have to remix.
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 the chilled dark chocolate cake, cut side up onto the cake board.
Add a thin layer of nutella buttercream over the cake and pipe a circle of buttercream around the edges of the cake.
Add 1/3 cup of marshmallow fluff filling inside the ring of buttercream. Then add a drizzle of melted nutella on top of the filling.
Place another cake layer cut side down over the top. Repeat the process for another layer — Crumb coat with nutella buttercream and freeze for 30 minutes.
Add a thick layer of nutella buttercream all over the cake. Use an offset spatula to fill in any gaps with more buttercream. Then use a cake scraper to smooth out the sides of the cake. Use the offset spatula to make the top edges sharp and smooth.
Add the white chocolate water ganache dripping down the sides and the top of the cake. Use a small offset spatula to smooth out the ganache on top.
Finally, add the large marshmallows on top and add the paper straw to complete this hot chocolate cake. Serve at room temperature and enjoy!