Preheat the oven to 180C/350F/Gas 4.
For the cake, grease and line the base of a 23cm/9in springform tin with baking parchment.
Blend the chocolate and sugar in a food processor until a fine powder forms. Add the boiling water, butter, egg yolks, coffee powder and vanilla extract and blend until well combined.
In a clean bowl, whisk the egg whites until stiff peaks form when the whisk is removed, then, using a metal spoon, gently fold into the chocolate mixture.
Pour the mixture into the prepared cake tin and bake for 45-55 minutes, or until a skewer inserted into the middle comes out clean.
Remove the cake from the oven and allow to cool in the tin, then transfer to the fridge for 2-3 hours.
To serve, remove the cake from the tin and place on a serving plate or cake stand, spread the crème fraîche on top, then scatter over the berries. To decorate, grate some chocolate over the top, or dust with icing sugar.
