Separate the eggs. Mix the flour with a pinch of tartar baking powder. Melt the chocolate with butter and sugar in a double boiler. Stir in the egg yolks and flour. Beat the egg white until stiff and fold in carefully.
Grease a springform pan well with butter and pour in the batter. Bake at 180 degrees in a preheated oven for 30 - 45 minutes (stick test).
Let it cool down well, then remove it from the mold. Sprinkle with powdered sugar and serve with fruit if necessary.