Separate the eggs. Beat the egg yolks with sugar until the mixture turns white. Melt the butter with the chocolate in a double boiler and let cool down a little. Beat the egg whites until stiff. Then stir the chocolate butter and flour into the egg yolk mixture, carefully fold in the egg white.
Pour the dough into a buttered and floured or sugared tin and bake at 160 ° C for 40 minutes.