Prepare the gelatin as directed on the box. Reserve.
In a pot over low heat, whisk together the milk, sugar, and gelatin until the gelatin melts. Do not boil at any time, or the gelatin will lose its properties. Remove from heat.
Melt the chocolate in the microwave, add the milk and gelatin mixture, and mix until well incorporated. Reserve and let the chocolate mixture cool a little bit.
With a handheld or stand mixer fitted with a whisk attachment, whisk the heavy cream on medium-high speed until stiff peaks form.
Whisk ¼ of the whipped cream into the cooled chocolate. Then gently fold in the remaining whipped cream.
Put the chocolate mousse in the fridge for about 1 hour or in the freezer for 20 minutes.