Melt the chocolate in a bain-marie with the butter. Beat the 5 yolks with the sugar and then add the flour, stirring in well, the melted chocolate (not too hot) and, last of all, the stiffly beaten egg whites.
Now pour the mixture into a buttered and floured cake tin and bake in a preheated oven, at 160 °C, for 30-40 minutes (1). This is a cake for those with a sweet tooth, crunchy on the outside and creamy inside (and you can make it in less than an hour!)
(1) Should the top of the cake begin to brown too much, halfway through the cooking time, quickly cover it with baking foil.