For the chocolate cake, begin by preheating the oven to 350F/177C convection (or 325F/162C conventional) and grease and line the base of four 6-inch (20cm) cake pans. 

In a medium bowl, whisk together the flour, granulated sugar, cocoa powder, baking soda, baking powder, espresso powder and salt. Set the dry ingredients aside.

In a large bowl, whisk together the eggs, oil, buttermilk, coffee and hazelnut extract until they’re well combined. Sift the dry ingredients over the wet ingredients and fold them together just until the last streak of flour is combined. 

Distribute the batter evenly among the prepared cake pans (about 11 oz per 6-inch pan). Bake for 30 minutes, until the center of the cake springs back when it's poked or a toothpick inserted comes out clean.

Remove the cakes from the pan and allow them to cool completely. 

Begin by thoroughly cleaning the aluminum bowl and the whisk attachment of the stand mixer (they can’t have any fat residue when making meringue). Once they’re clean and dry, place the egg whites and the granulated sugar in the bowl.

In a small saucepan, bring 1/2 cup (118mL) of water to a simmer. Keep the water simmering on low heat and place the bowl with the egg whites and sugar on top of the saucepan, creating a double boiler. Whisk the egg whites and sugar over low heat for 5 to 10 minutes, until the sugar granules are dissolved. You can feel it between your fingers if the sugar is fully dissolved or you can use a candy thermometer and bring the mixture to about 140°F (60°C).

When the sugar granules are fully dissolved in the egg whites, remove the bowl from the heat and place it on the stand mixer fitted with the whisk attachment. Beat the mixture on medium-high speed, until stiff peaks form and the meringue no longer feels warm. This usually takes about 10 minutes.

Meanwhile combine hot water with espresso powder and mix them until it's dissolved. Set it aside to cool.

Slowly add in the softened butter to the meringue, 2 to 3 tablespoons at a time. Once the butter is incorporated, continue beating on medium speed for a few minutes until it emulsifies into a buttercream. It may go through phases of looking soupy and/or curdled before it emulsifies but it should come together within 5-10 minutes. 

After the buttercream comes together, add in the espresso mixture and the hazelnut extract and mix until they're fully combined.

If you feel like the cake is getting unstable at any point, place it in the refrigerator for about 30-60 minutes, then finish frosting it. The buttercream gets firm in the fridge so make sure to gently smooth out the edges if any buttercream squeezes out between the layers.

When the cake is fully stacked, refrigerate for 1-2 hours and then finish frosting the top and sides of the cake. Chelsweets has a great tutorial on how to frost a cake if you've never frosted a layered cake before.

To get the design that I did, simply place a small spoon or a small offset spatula against the frosted cake and then slightly move it upwards as you're spinning the turntable. Do the same with the top of the cake but start on the outside and move inside. Top with chocolate shavings and toasted hazelnuts.