Preheat the oven to 350°F conventional (no fan) or 325°F with the fan. Grease and line your chosen pans with parchment. I used 3, 8 inch round baking pans.

Sift together the flour, cocoa powder, baking soda, baking powder, and salt. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar on medium speed until light and fluffy.

Add the eggs one at a time, scraping down the sides and the bottom of the bowl well after each addition. Add the egg yolk along with one of the other eggs.

In a large measuring cup or container, combine buttermilk, vanilla, and coffee. Alternately add about a third of the sifted dry ingredients with about ½ the liquids. Repeat with another third of the dry ingredients and the remaining wet ingredients, and finally add the remaining flour mixture. Mix just until incorporated. I like to finish folding in the dry ingredients with a large rubber spatula.

Divide the batter into prepared pans (approximately 540g each for 3 cakes) and bake until a cake tester comes out with a few clinging crumbs and the center provides a little resistance when pressed lightly with your finger.

Allow the cakes to cool for 5 minutes in the pan, then flip them out onto a wire rack and remove the paper from the bottom of each cake. Allow to cool completely before flipping back over. Don’t stress if a little bit sticks to the rack.

Heat about an inch of water up in a medium sauce pot until it is just about simmering. The most important thing is that the bowl of your stand mixer should fit inside so that it is “nested” but the water does not touch the bottom of the bowl.

Whisk together the egg whites and sugar. Then place the bowl over top of simmering water and continue whisking constantly. Cook the whites and sugar just until it reaches 120°F on an instant-read thermometer or until the sugar has dissolved and the mixture is warm to the touch. The meringue will have started to gain a bit of volume and feel lighter at this point as well.

Move the bowl to your stand mixer and fit it with the whisk attachment. Be careful and use a towel, because the bowl will be hot if you're using a gas burner! Beat on high speed until the meringue reaches stiff peaks and cools to below body temperature. This will only take 2-4 minutes, depending on your mixer.

Switch to the paddle attachment and then add all the butter. Mix on medium-low until all the butter works itself into the meringue and a silky smooth buttercream forms.

Once your buttercream comes together, stop the mixer, add all the chocolate, and mix on medium speed to incorporate. Switch back to the whisk and beat it up until it is light and fluffy!

Once your cake is completely cool, level each layer with a serrated knife using a sawing motion. We aren’t in a fancy bakery, so just take the mound off the top rather than getting it perfectly even with the sides. It just kills me to waste that much! Place one layer on a cake plate, cake board on a turntable, or directly on a cake stand if you are bold.

Scoop a generous cup and a half of frosting onto the top of the first layer, then spread it to the edges using a large offset spatula. Place the second layer on top of the frosting and then refrigerate for 20-30 minutes.

Repeat with the second layer of frosting, then place the third layer upside-down on top. This gives you a perfectly flat top, which will make perfect edges easier if you care.

Depending on the state of your buttercream, I like to give my cake a quick crumb coat and then refrigerate it for 30 minutes to an hour. If your buttercream is too soft and is squishing out when you try to frost the top, refrigerate it until the cake is firm.

Add another cup to the top of the layer of the cake and spread it out evenly to the edges, and begin to spread any excess over the sides. Using your offset spatula, add frosting to the sides of the cake and spread it out evenly, into all the gaps between the layers and down to the bottom of the cake plate.

Turn the cake as you work and continue adding buttercream until the whole cake is covered and is the thickness that you desire. With this buttercream, I like a ¼ inch thickness because when served at room temperature, it is lighter than air and is the perfect ratio. That would be too thick if you were using a chocolate buttercream frosting instead.

Smooth the edges using the edge of a metal bench scraper (my favorite), a large offset spatula or a plastic ruler. You can leave the top edges “torn” or you can use your favorite tool to smooth the top and square off the edges gradually.

Refrigerate for at least an hour until set before decorating. If you would rather do an artful swirl, then it is best to swirl with the back of a spoon or the tip of your offset spatula while the buttercream is still soft.