How to Make It
Preheat the oven to 350°. In a food processor, combine the cookie crumbs with the hazelnuts and sugar and pulse until fine crumbs form. Add the butter and process until incorporated. Press the crumbs evenly over the bottom and up the side of a 9-inch glass pie plate. Bake the crust for 8 minutes, or until set. Using a flat-bottomed glass, lightly tamp down the crust and let cool.
Put 2/3 cup of the chocolate in a large bowl. Put the remaining chocolate in a small bowl. In a saucepan, heat the cream until bubbles appear around the edge of the pan. Spoon 2 tablespoons of the cream into the small bowl of chocolate to melt it; stir in 2 tablespoons of the chestnut puree. Set the ganache aside.
Pour the remaining 1 cup of hot cream into the large bowl to melt the chocolate. Whisk in 2/3 cup of the chestnut puree and the salt. Refrigerate the chocolate-chestnut mousse mixture until very cold, about 2 hours.
Using an electric mixer, beat the mousse until soft peaks form. In a glass bowl, sprinkle the gelatin over the water and let soften. Microwave for about 8 seconds on high to melt the gelatin. Beat the gelatin into the mousse. Stir the remaining plain chestnut puree and spread it in the cookie crust. Spread the mousse on top and freeze until firm.
Rewarm the ganache in the microwave for 10 seconds, then spread it over the frozen mousse. Freeze until just set. Refrigerate the pie for 10 minutes before cutting into wedges and serving.