Special equipment: an 8-inch springform pan
Bring the cream, sugar and vanilla to a low boil in a large saucepan.
Meanwhile, pour the milk into a pie plate or shallow bowl and sprinkle with the gelatin in an even layer. Let sit until the gelatin looks completely damp, like wet sand.
When the cream just reaches a boil, remove it from the heat and immediately add the chocolate, stirring to mix through. Let sit for a few minutes to melt the chocolate.
Lightly coat an 8-inch springform pan with nonstick spray. Pulse the cookies in a food processor until very fine, almost powdery. Add the cookie crumbs to a large bowl and mix in the butter with a wooden spoon until the mixture holds together when pinched. Transfer the crust mixture to the prepared springform and pat evenly into the bottom and sides of the pan. Set aside.
Whisk the hot cream and melted chocolate until smooth. Add the gelatin mixture, stirring well to melt the gelatin. Let cool slightly before pouring the filling through a sieve into the crust. Freeze for at least 2 hours or overnight. (To freeze for more than a day, wrap the entire springform in plastic wrap and then foil, and freeze for up to 1 month.)
Top with whipped cream before serving.