I am including this pie for my brother, who requested this dessert for every family event from the time he was tiny until he entered his 20s and lost his taste for sweets (a strange, inexplicable event in our family that I feel validates my childhood taunting that he was adopted). If you feel like a milk chocolate mousse, substitute milk chocolate for the bittersweet.
In a large, heatproof bowl or the top pan of a double boiler, combine the chocolate, butter, coffee, and rum. Set over (not touching) barely simmering water and stir constantly until melted and smooth. Remove from the heat and let cool for 15 minutes.
In a stand mixer fitted with the whip attachment, combine 1 ½ cups (360ml) of the cream and the vanilla and whip on medium-high speed until soft peaks form. (Or, use a large bowl and a handheld mixer or a whisk.)
Fold 1/3 of the whipped cream into the chocolate mixture to lighten it. Then, fold in the remaining whipped cream just until combined. It's better to have some streaks of cream in your mousse then to stir the mixture until it loses its airiness. Cover the mousse and refrigerate for at least 2 hours or up to 3 days.
Line six ½-pint (24-ml) jars with the crumb crust. Whip the remaining ½ cup (120 ml) cream until soft peaks form. Spoon about ½ cup (120 ml) mousse into each crust-lined jar. Top the mousse with a dollop of the whipped cream and grate or shave a little chocolate on top or dust with cocoa powder.
Serve immediately, or cover and refrigeratre for up to 3 days. These pies can also be frozen for up to 1 month without the cream topping. Bring to room temperature and add the cream topping just before serving.