Preheat the oven to 360 F. Beat soft butter with brown sugar in a bowl. Separate the whites from the yolks. Add the yolks one at a time into this bowl. Mix. Sift flour mixed with cocoa powder, add into the bowl, as well as coffee. Stir with a spatula or mixer.
Beat egg whites to strong peaks in a separate bowl. Gently fold the beaten egg whites into the chocolate batter. Line a 7" baking dish with parchment paper and transfer the chocolate batter into it. Bake until toothpick comes out dry with moist crumbs attached, about 25-30 minutes.
It is best to let the cake cool slightly right in the pan, after which it can be easily removed, as it will decrease in size. Let the cake cool, run a knife along the outside to loosen. Remove, then cut it into two layers. If the top has risen and cracked, it is better to cut it off too so both layers are flat.
Whip the whipping cream with cocoa powder and sugar (about 1 minute if the cream is 50% fat, and 2 minutes if it is 35%).
It is convenient to use a springform pan to assemble the cake. Place the first layer of cake down and spread with the whipped cream.
Put the second layer of cake on top. Refrigerate for an hour.
Put dark chocolate pieces in a bowl, add cream and butter. Heat in the microwave for 30 second increments 2-3 times until smooth and melted.
Remove cake and place on a plate. Spread the top and sides of the cake with chocolate ganache and refrigerate until completely set.