In a small bowl, combine gelatin and cold water and bloom for 5-10 minutes.
Meanwhile, place chopped chocolate in a heatproof bowl. In a small saucepan set over medium-high heat, bring 1 cup of heavy cream to a simmer (between 112-115 degrees F), then remove from heat.
Whisk in bloomed gelatin and stir until completely dissolved. Pour mixture over the chopped chocolate and whisk until melted and combined. Allow mixture to cool, stirring occasionally.
In the bowl of a stand mixer fitted with a whisk attachment, or with a hand mixer in a large bowl, beat remaining 2 cups cream with powdered sugar until stiff peaks form.
Stir ~1/4 cup of the whipped cream into the melted chocolate. Gently fold in the remaining whipped cream until fully incorporated.
Spread the chocolate mousse evenly on top of the cooled cake while still inside the pan.
Cover with plastic wrap and refrigerate overnight.