Toast the hazelnuts in a dry pan until golden, then leave to cool slightly and blitz to a fine consistency in a food processor. Remove the cardamom seeds from their pods and grind using a pestle and mortar.
Heat oven to 160C/140C fan/gas 3. Grease and line the base of a 23cm spring-form cake tin. Use a microwave to melt the chocolate with the butter in 30-sec bursts until glossy and smooth. Leave to cool slightly.
Using an electric whisk, in a very clean bowl whisk the egg whites until they reach stiff peaks. Then, without cleaning the beaters, whisk the yolks with the sugar in a separate bowl until pale and voluminous.
Combine the chocolate with the egg yolk mixture, then incorporate the cocoa powder, a pinch of salt, the cardamom seeds and hazelnuts. Add a spoonful of egg white to the batter, stirring it through to loosen the mix, then fold in the rest, taking care to keep in as much air as possible. Gently pour into the tin and bake for 35 mins. Leave to cool in the tin, then dust with cocoa powder and serve with crème fraîche.