Soften gelatin in 1/4 c. cold water. In double boiler, cook and stir milk and chocolate until smooth and thick (about 5 minutes). Gradually stir in 1/2 c. water, keeping mixture smooth.
Beat egg yolk slightly. Stir small amount of hot mixture into eggs and add eggs to double boiler. Cook and stir until thick and smooth, about 5 minutes.
Remove from heat and stir in vanilla, gelatin, and cognac until gelatin is dissolved. Cool to room temperature.
Beat egg whites until stiff. When chocolate mixture has cooled so that it mounds slightly when dropped from spoon, fold in egg whites. Refrigerate for 3 hours and serve, garnished with whipped cream.