If not already baked, bake the pie crust to package directions and let cool while you make the chocolate mousse filling.
Combine the chopped chocolate, vanilla, and salt in a heatproof bowl.In a small saucepan, bring ¾ cup of the heavy cream to a boil. Pour the cream over the chocolate, salt, and vanilla.
Stir gently for several minutes until the chocolate melts and combines with the heavy cream. It will be lumpy and weird at first but keep stirring until the mixture becomes smooth, glossy, and evenly combined. Let the chocolate mixture sit to cool slightly while you whip the cream, stirring every once in awhile so it doesn’t harden.
While the chocolate ganache is resting, whip the cream with the sugar (with a whisk or whisk attachment on a mixer) until stiff peaks form. Don’t overbeat – once the cream separates you’re making butter and need to restart.
When the chocolate mixture has just barely cooled and the cream is whipped, very gently combine about a cup of the whipped cream with the chocolate mixture. Then, combine the chocolate/cream mixture with the rest of the whipped cream (in whichever is the larger bowl), again working very gently until the mixture is combined but still very fluffy.
Scoop the chocolate mousse into the prepared pie crust. Refrigerate for at least 3 hours until set. This is a fabulous dish to make the night before serving to save time and dishes on the day of an event!