Beat the heavy cream at medium-high speed in a mixing bowl until it forms whipped cream, then transfer it to another bowl.
Let the melted chocolate cool slightly, then gently fold it into the whipped cream with a spatula until fully combined.
Transfer the mousse into a piping bag and fill serving glasses.
Top each glass with crushed cookies, decorate a whole cookie as a 'gravestone' using black pastry gel, and insert it into the mousse.