Melt butter or margarine in a large saucepan, remove from the hob and let the chocolate, broken into pieces, melt in the warm fat. Add sugar and egg yolks and stir vigorously with a wooden spoon. Mix the coffee and flour and stir into the chocolate mixture. Beat the egg whites until stiff and stir carefully into the batter.
Put in a greased springform pan (22 cm). Bake immediately on the lowest rack for about 30 minutes. The cake must still be a bit wet and sticky and often cracks on the top.
Cover with chocolate icing, depending on your taste.