Preheat the oven to 160°C. Line a 18 cm round cake tin with parchment paper and lightly grease the sides.
Break the chocolate into pieces and place it into a bowl set over simmering water. When the chocolate has melted, add the butter and muscovado sugar. Stir until the butter has melted and the muscovado sugar has dissolved. Take off the heat.
Finely grind the almonds and hazelnuts using a spice grinder. It’s perfect if small pieces of nut remain. Add to the melted chocolate and mix thoroughly.
Beat the egg whites until firm.
Add the egg yolks to the chocolate mix in a stream, stirring constantly to combine. Add a third of the egg white, fold through, then add the rest, continuing to fold.
Pour the cake mix into the pan. Bake for 45 minutes, then turn off the oven and leave the cake in the oven until completely cooled.
Sift cocoa powder onto the surface of the cake with a fine wire mesh before serving.