Break the chocolate into small pieces and melt in the microwave with the butter on 50% power in 30 second intervals, stirring between each (you can also do this over a double boiler).
Place the chocolate in a large mixing bow then add the flour and mix well. In a separate bowl combine the egg yolks with the 200g sugar and beat until very fluffy. Slowly beat into the chocolate.
Beat the egg whites with the 50g sugar until stiff and fold carefully into the chocolate mixture.
Line the base of a springform cake tin with parchment paper. Grease the paper and tin walls. Pour in the batter and bake 30-35 minutes. Let cool.
Mix the raspberries with 2 tablespoons sugar and let sit for 45 minutes. Decorate the top of the cake with the berries then sprinkle with icing sugar just before serving.