Lightly grease a 9-inch springform pan and line the bottom with parchment paper if desired.
Crush the chocolate sandwich cookies into fine crumbs. Mix with the melted butter until evenly moistened.
Press the cookie mixture firmly into the bottom of the prepared pan. Refrigerate for 15 minutes.
Place the cold water in a small bowl and sprinkle the gelatin over it. Let it soften for about 5 minutes.
Combine the raspberries, granulated sugar, and lemon juice in a small saucepan. Cook over medium heat for 5 to 7 minutes, stirring occasionally, until the berries soften.
Strain the raspberry mixture through a fine-mesh strainer if you prefer a seedless layer. Stir in the prepared gelatin until fully dissolved.
Set the raspberry mixture aside to cool until it is no longer hot but remains pourable.
Place the chopped chocolate in a heatproof bowl. Heat 1/2 cup of the heavy cream until steaming, then pour it over the chocolate. Let stand for 2 to 3 minutes before stirring until smooth
Stir in the vanilla extract and allow the chocolate mixture to cool to room temperature.
Beat the remaining 1 cup heavy cream with the powdered sugar until soft to medium peaks form
Gently fold the whipped cream into the cooled chocolate mixture in several additions until the mousse is smooth and fluffy.
Spread half of the chocolate mousse over the chilled cookie crust.
Carefully spread the cooled raspberry mixture over the mousse.
Add the remaining chocolate mousse and smooth the top with an offset spatula.
Cover and refrigerate for at least 6 hours, preferably overnight.
Before serving, decorate with whipped cream, fresh raspberries, and chocolate shavings if desired.
Run a warm, dry knife around the edge of the cake before releasing the springform pan. Slice with a warm, sharp knife for clean layers.
