Take a large bowl and pour in the heavy whipping cream. Whip it on medium-high speed until it becomes thick and forms stiff peaks. This should take about 5 minutes.
In another large bowl, whisk the chocolate pudding mix and cocoa powder until completely combined. Add the cold milk and whisk for 2 minutes.
Carefully fold the whipped cream into the chocolate pudding mixture until there are no more white streaks left.
Put the mousse into desired serving glasses and serve immediately, or place it in the fridge until ready to enjoy!