Chocolate Celebration Cake

I was given this recipe by a friend who is a fabulous cake maker - Anna Castillo.  Because it was a weekend of numerous birthday celebrations I thought it was the perfect opportunity to whip this up and it did not disappoint! 

150 grams dark chocolate (chopped)
150 grams soft unsalted butter (plus some for greasing)
6 large eggs
200 grams caster sugar (I use rapadura)
100 grams ground almonds
4 teaspoons best-quality cocoa powder (sifted)
Double espresso shot (can be omitted)
Icing sugar or cocoa powder (to dust)
Preheat the oven to 180°C, line the base of a 23cm  springform cake tin with baking parchment and butter the sides.
Melt the chocolate and butter together either in a heatproof bowl, Bain Marie, then set aside to cool slightly.
Beat the eggs and sugar together until about tripled in volume, pale and moussy.
Mix the ground almonds with the cocoa powder and fold this gently into the egg and sugar mixture, followed by the slightly cooled chocolate and butter. Finally, fold in the espresso.
Pour and scrape this mixture into the prepared tin and bake in the preheated oven for 40–45 minutes (though start to check at 35); the cake will be just firm on top, but still have a bit of wobble underneath.
Remove from the oven and sit the cake in its tin on a wire rack to cool. Once the first heat has left it, drape a clean tea towel over the cake to stop it getting too crusty, though a cracked and cratered surface is to be expected.
When cold, unmould, dust with icing sugar or cocoa powder.