Preheat the oven to 350°F. Grease and line 3 8-inch rounds, or you can stagger cake sizes for a more tiered effect like the cake picture.
In the bowl of a stand mixer, beat the egg whites until stiff peaks form. Set aside.
In a graduated measuring cup, whisk together the cacao and espresso/coffee until smooth. Set aside.
In a heatproof bowl over a small saucepan of simmering water, melt the chocolate and butter, watching carefully and stirring to ensure they do not burn. Remove from heat. Add the espresso mixture and the brown sugar, and whisk to combine.
In the stand mixer, use the whisk attachment to combine the almond flour, hazelnut flour, and egg yolks. Gently fold in the egg whites, in three additions.
Divide the batter between the prepared pans.
Bake until a cake tester comes out clean with a few crumbs, about 45 minutes. Transfer to a wire rack to cool before removing from the pans. Meanwhile, prepare the frosting.