In a cold bowl whip heavy cream to a thickened consistency (once the whip leaves thick marks into the cream it has been whipped enough); do not whip cream as thick as you would to serve with pies or other desserts. Whipping cream too stiff will result in lesser final volume and a heavy textured chocolate mousse.
In a small cup combine gelatin and water and set aside for 5 minutes.
Bring milk to a boil and remove from heat. Whisk in the gelatin followed by the chocolate. Whisk until the mixture is very smooth. Pour mixture into a bowl large enough to hold chocolate mixture and whipped cream.
Using a rubber spatula gently fold in cream. Do not over-fold as this will cause the mousse to become heavy.
Pour mousse into serving bowls or glasses. Place in refrigerator.
Allow to set for several hours.
If desired, grate a small piece of chocolate on a cheese grater and sprinkle on surface.