Preparation

• Butter a 9-inch springform pan.
• Grind the cookies in a food processor until they resemble wet sand, 20 to 30 seconds; you will have about 1-3/4 cups. Transfer to a small bowl and mix in the butter. Spread the crumbs in the pan, cover with plastic wrap, and press evenly into the bottom. Refrigerate.
• Combine the chocolate, 1/2 tsp. of the vanilla, and the salt in a large bowl. In a small saucepan, bring 3/4 cup of the cream to a bare simmer. Pour the cream over the chocolate, let sit for 1 minute, then whisk until smooth. Cover and refrigerate for about 30 minutes to cool.
• Beat 1-1/2 cups of the cream in a medium bowl with an electric mixer on medium-high speed to stiff peaks, about 2 minutes. Whisk the chocolate mixture to loosen it, and fold it into the whipped cream with a large silicone spatula until no streaks remain.
• Carefully peel the plastic wrap off the crust and scrape the mousse into the pan, gently spreading it to the edges. Cover and refrigerate for at least 6 hours.
• Just before serving, beat the remaining 1-1/2 cups cream and 1 tsp. vanilla in a medium bowl to medium-stiff peaks. Run a knife around the pie to loosen its edges and then remove the side of the pan. Slide a spatula under the crust and transfer the pie to a serving plate. Mound the whipped cream over the mousse and top with chocolate curls, shards, or shavings. To serve, dip a knife into hot water and dry it before slicing.

Make Ahead Tips

You can make the crust and fill it with the mousse up to 2 days ahead. Refrigerate, covered. Whip the cream for the topping and top the pie just before serving.