• Lightly butter the bottom and sides of a 9-inch springform pan that is at least 3 inches deep. Set it aside while you prepare the crust.

Make the crust

• In a medium bowl, stir the cookie crumbs together with the melted butter until every crumb is evenly moistened. Press the mixture firmly and evenly over the bottom of the prepared pan. Place the pan in the fridge to chill while you prepare the peanut butter mousse.
• If you would like a firmer, crunchier crust, you can bake it first. Place it in a preheated 350°F, 180°C oven for 7 to 8 minutes, then transfer it to a wire rack and let it cool completely before adding the mousse.

Make the peanut butter mousse

• Place the cream cheese in the bowl of an electric mixer fitted with the paddle attachment. Beat over medium-high speed until light and fluffy. Add the softened butter and continue mixing until smooth and creamy, about 1 minute.
• Add the powdered sugar and beat until the mixture is fluffy and well combined. Mix in the peanut butter and salt, then set this mixture aside.
• In a clean mixer bowl fitted with the whisk attachment, or using a handheld mixer, whip the heavy cream until soft peaks form. The cream should hold its shape while still looking soft and smooth.
• Using a rubber spatula, gently fold 1 cup of the whipped cream into the peanut butter mixture. Once incorporated, fold in another 1½ cups of whipped cream.
• Keep the remaining whipped cream covered in the fridge. You will need it shortly for the chocolate mousse.
• Spoon the peanut butter mousse over the chilled crust and gently spread it into a smooth, even layer. Return the cake to the fridge while you prepare the chocolate mousse.

Make the chocolate mousse

• Place the bittersweet chocolate, milk chocolate, and milk in a microwave-safe bowl. Heat in 30-second intervals, stirring well between each one, until the chocolate is completely melted and smooth. You can also melt the chocolate mixture gently in a double boiler on the stove.
• Stir in the vanilla extract and leave the chocolate mixture to cool for about 5 minutes.
• Take half of the remaining whipped cream and gently fold it into the cooled chocolate mixture using a rubber spatula. Add the rest of the whipped cream and fold gently until the mousse is smooth and evenly combined.
• Pour the chocolate mousse over the peanut butter layer and carefully smooth the top. Cover the cake loosely and place it in the freezer or fridge for 1 hour.

Make the chocolate ganache glaze

• Place the chocolate and cream in a microwave-safe bowl. Heat in 30-second intervals, stirring between each round, until the chocolate has melted completely and the glaze is smooth.
• Pour the ganache over the chilled cake and gently spread it across the surface with a knife, creating an even layer.
• Cover the pan loosely with plastic wrap and return the cake to the fridge. Let it chill for at least 6 hours, or until the layers are completely set. For the cleanest slices, give it plenty of time to firm up.

Release and serve

• When you are ready to serve, run a knife under hot water, then dry it well. Carefully slide the warm knife between the cake and the inside edge of the springform pan to loosen the cake. Slowly release and remove the sides of the pan.
• Serve the chocolate peanut butter mousse cake cold.
• Keep any leftover cake covered in the fridge, where it will stay fresh for up to 5 days.