A decadent dessert with three layers of rich chocolate cake with Nutella filling, Nutella cream cheese frosting, dark chocolate ganache, and chocolate hazelnut truffles.
Preheat oven to 315 degrees Fahrenheit and prepare three six-inch cake pans by lining the bottoms with parchment paper. Then, grease and flour the pans.
In a medium size bowl, whisk together the flour, cocoa powder, espresso powder, baking powder, baking soda, and salt. Set aside.
In the bowl of a stand mixer (or a large bowl with a hand mixer), beat together both sugars and the oil until well incorporated. Add vanilla extract, then the eggs and egg yolk one at a time.
With the mixer on low speed, alternate adding 1/3 of the flour mixture and 1/2 of the buttermilk, starting and ending with the flour and mixing until incorporated after each addition. You will have three additions of flour and two additions of milk.
Add hot water and fold into batter by hand with a spatula or wooden spoon.
Divide batter evenly between three prepared cake pans. Tap pans gently on the counter to remove excess air bubbles. Bake at 315 degrees for 37-40 minutes, or until a toothpick inserted into the center of the cake comes out with just a few moist crumbs attached.
Let cakes cool in pans for 5-10 minutes, then turn out onto a wire rack and let cool completely.
Using an electric mixer, beat together the butter and cream cheese on medium speed until light and fluffy. And the Nutella and beat until well combined.
With the mixer on low-speed, add the powdered sugar gradually (about 1 cup at a time), mixing until incorporated after each addition. Then, increase speed to medium-high and beat until smooth.
Fit a piping bag with a large round tip and fill with some of the frosting.
Once cakes are completely cool, use a serrated knife to slice off the tops of each layer, to create a completely level layer. Place bottom layer on a cake board, using a small amount of buttercream to secure.
Pipe a dam (or a ring) of frosting around the outer edge of the cake (this will keep the Nutella inside). Then, fill in the middle with about 1/4 cup of the Nutella** and spread evenly with an offset spatula. Top with second layer and repeat this step, finishing by topping with the third layer (cut side down).
Frost the top and sides of the cake with a very thin layer of frosting (a crumb coat) to lock in the crumbs and even out the sides. Chill cake for at least 20 minutes.
Reserve 1/2 or 3/4 of a cup of frosting (for piping on top), then use what's left to finish frosting the cake completely. Chill cake again for about 20 minutes. Meanwhile, make the chocolate ganache.
Place chocolate chips in a heatproof bowl. Heat the heavy whipping cream in a small saucepan on medium-low heat until the cream is heated through and small bubbles begin to form around the edges.
Pour cream over the chocolate chips and let stand for 5 minutes. Then, whisk until smooth and let cool for a few minutes. Ganache should slowly, but fluidly drip off a spoon for the correct consistency.
Remove cake from fridge and spoon the ganache around the edges of the cake, pushing it over the edges to drip down the sides of the cake as desired. Smooth the top with the back of a spoon or an offset spatula.
Using a large, closed star piping tip, pipe swirls around the outer edge of the cake and top each swirl with a chocolate hazelnut truffle.
*substitute 1/2 cup milk with a teaspoon and a half of lemon juice OR white vinegar for buttermilk. Stir together and let sit for 5 minutes before using.
**If Nutella is too firm to spread evenly on your cake, put the Nutella in the microwave for 10 seconds at a time until it is spreadable.