Melt the chocolate with the coffee in the top part of a double boiler over hot water.
Melt the butter in a saucepan over low heat. Blend in the flour. Gradually add the hot milk, sugar and salt. Cook, stirring constantly, for about five minutes, until the mixture is smooth and thickened.
Blend in the chocolate-coffee mixture and the beaten egg yolks. Cool.
Fold in the stiffly beaten egg whites.
Butter six individual soufflé dishes and sprinkle with sugar; or butter and sugar a one-quart soufflé dish. Turn the mixture into the soufflé dishes, filling them two-thirds full.
Bake the individual soufflés in a preheated moderate oven (350°F.) for ten to fifteen minutes. Bake the one-quart soufflé at 375°F. for thirty to forty-five minutes.