Put gelatin in a small bowl. Pour in 1/4 cup milk and let stand for 5 minutes. Put the chocolate and 1/2 cup milk in the top part of a double boiler; heat over hot water. When the chocolate is melted, beat vigorously until smooth. Stir soaked gelatin and the sugar into the chocolate mixture; stir until sugar and gelatin are dissolved. Let cool. Prepare cream by beating with a mixer until smooth and thick. Fold in vanilla. When gelatin mixture begins to thicken, fold it into the whipped cream. Divide into serving dishes and freeze or refrigerate until serving.