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.
Type the ingredients you want to use, then click Go. For better results you can use quotation marks around phrases (e.g. "chicken breast"). Alternatively you can search by chef, programme, cuisine, diet, or dish (e.g. Lasagne).