Preheat oven to 170 degrees C. Grease and flour a 20cm springform pan. In the top of a double boiler, heat chocolate and butter, stirring occasionally, until chocolate is melted and smooth. Remove from heat and allow to cool to lukewarm.
Transfer chocolate mixture to a large bowl. Beat in the flour and egg yolks. In a separate large glass or metal mixing bowl, beat egg whites until foamy. Gradually add sugar, continuing to beat until stiff peaks form. Fold 1/3 of the whites into the chocolate mixture, then quickly fold in remaining whites until no streaks remain. Pour batter into prepared pan.
Bake in the preheated oven for 35 minutes. Turn off oven, crack the door, and allow cake to cool in oven.
In a saucepan, melt Mars Bars with the cream. Pour over cooled cake.