Melt butter in a small saucepan and pour into a mixing bowl. Add 5 egg yolks (reserving egg whites in a separate bowl) and caster sugar and whisk together into a thick cream. Add flour and baking powder and mix all together until smooth.
Break chocolate into pieces and melt slowly in small saucepan over low heat. Remove from the heat and slowly stir into egg mixture.
Add a pinch of salt to egg whites in bowl and beat until stiff peaks form. Fold gently into the egg chocolate cream.
Preheat oven to 190 C / Gas 5. Lightly grease a 23cm round cake tin.
Pour cake mixture into tin and bake in the preheated oven until a skewer inserted in the centre comes out clean, about 30 minutes. Let cool 1 hour before unmoulding from tin.
Melt chocolate in small saucepan over medium low heat. Mix in icing sugar and crème fraîche and cook over low heat for 5 minutes. Add egg yolk and stir together until smooth. Remove from heat and spread icing over cooled cake.
Garnish with chocolate sprinkles and refrigerate for at least 3 hours until cake is well chilled.