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.
nutrition information (per serving):
38, Fat Calories
350, Saturated Fat
4, Monounsaturated Fat
30, Polyunsaturated Fat
Photo: Scott Phillips