Chocolate shavings, for garnish, optional
Preheat the oven to 375 degrees F.
Place the chocolate wafer cookies in the bowl of a food processor and process until fine crumbs. Add the sugar and butter and process until thoroughly combined. Transfer the crumb mixture to a 9 1/2-inch pie pan and press firmly onto the bottom and up the sides to form a crust. Bake for 8 to 10 minutes, or until fragrant. Transfer to a cooling rack and allow to cool completely.
In a small bowl combine the rum, water and gelatin. Stir and set aside until gelatin has softened, about 5 minutes.
In a saucepan combine the milk, egg yolks, sugar and cornstarch and stir or whisk until thoroughly combined. Heat over medium-high heat until mixture comes to a boil and thickens, whisking constantly. Remove from the heat and add the gelatin mixture. Stir until smooth, then transfer 1 cup of the hot mixture to a clean mixing bowl and add the chocolate morsels. Stir until the chocolate is melted and the mixture is smooth. Transfer the chocolate mixture to the cooled crust and refrigerate, lightly covered, for 30 minutes. (Allow the remaining warm custard mixture to continue cooling on the counter in the interim.)
After 30 minutes have passed, add the heavy cream and confectioners' sugar to a medium mixing bowl and beat until stiff peaks form. Remove 1 cup of the whipped cream and add it to the remaining custard mixture. Fold to combine, then spread gently on top of the first layer in the pie crust. Top with the remaining whipped cream and refrigerate until thoroughly chilled and set, at least 4 hours and up to overnight.
When ready to serve, garnish the pie with chocolate curls and cut into slices. Serve cold.