Melt butter over medium heat and stir in the flour until well-blended. In a separate saucepan, heat but do not boil the milk, chocolate, and sugar. Add the hot milk mixture to the flour mixture, stirring constantly until well blended. Beat the egg yolks until light. Beat part of the sauce into the yolks, then add the yolk mixture to the rest of the sauce; stir the custard over low heat to thicken slightly. Add the vanilla. Cool. Whip the egg whites until stiff but not dry. Beat one-third of the egg whites into chocolate mixture. Then fold remainder of egg whites lightly into mixture. Use a straight-sided, 1-quart ovenproof baking dish. Grease the bottom and sides well with butter and then coat the buttered surfaces with a thorough dusting of powdered sugar. Set the souffle dish in a pan of hot water. Pour in souffle mixture, and bake about 20 minutes, or until firm. Serve at once with whipped cream. This is a recipe that you put in the oven after you start to eat. Let the guests wait for the souffle; do not try to keep souffle waiting for guests.