Preheat oven to 350F, grease three 6" round baking pans and dust with cocoa powder. Line bottoms with parchment.
Place all dry ingredients into the bowl of a stand mixer fitted with a paddle attachment. Stir to combine.
In a medium bowl whisk all wet ingredients (pour coffee in slowly as not to cook the eggs if very hot).
Add wet ingredients to dry and mix on medium for 2-3 mins. Batter will be very thin.
Pour evenly into prepared pans. I used a kitchen scale to ensure the batter is evenly distributed.
Bake until a cake tester comes out mostly clean. A total of 30-35mins.
Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.*
Place bowl over a pot with 1-2" of simmering water and stir constantly with a whisk until the mixture is hot and no longer grainy to the touch or reads 160F on a candy thermometer (about 3mins).
Place bowl on your stand mixer and whisk on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins))
Switch to paddle attachment (optional). Slowly add cubed butter and mix until smooth.
Add vanilla, melted chocolate, and espresso powder. Whip until smooth.**
Finely chop chocolate and place into a bowl. Bring cream just barely to a simmer and pour over chopped chocolate. Cover with plastic wrap and let stand 2 mins. Stir gently with a spatula until combined and smooth. Allow to cool and thicken slightly before using on cake.***
Place one layer of cake on a cake stand or serving plate. Top with approximately 2/3 cup of frosting and spread evenly. Repeat with remaining layers and apply a thin coat of frosting all over the cake. Chill for 20mins.
Frost and smooth the sides. Chill for 20mins.
Using a small spoon, place dollops of ganache around the top edges of the cooled cake, allowing some to drip down. Fill in the top of the cake with more ganache and spread evenly with an offset spatula.
*Ensure there is NO trace of egg yolks in your whites and that your mixer bowl and whisk is completely grease free or your meringue won't stiffen. ** The buttercream may look like it's curdled at some point. Keep mixing until it is completely smooth. *** You can make the ganache in advance and let it set overnight (with plastic wrap placed directly on the surface). It will be too thick at this point, so I microwave it for 5-10 secs and stir it until it's the right consistency.