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.