Preheat oven to 350°F (175°C). Generously grease a 9-inch springform pan with high sides.

In a bowl, mix the crushed cookies with softened butter until evenly combined.

Press the mixture firmly into the bottom of the prepared pan.

Bake the crust for 5 minutes, then remove and allow to cool completely.

In the top of a double boiler, combine 1 cup heavy cream, chocolate chips, vanilla, and salt. Heat gently until the chocolate is fully melted and smooth.

Remove from heat and transfer the chocolate mixture to a bowl. Let cool to room temperature, stirring occasionally.

In a separate bowl, whip 2 cups chilled heavy cream with 1/4 cup sugar until stiff peaks form.

Gently fold the whipped cream into the cooled chocolate mixture until fully incorporated.

Pour the mousse into the cooled crust and smooth the top.

Refrigerate for at least 6 hours, or until fully set.

Just before serving, whip the remaining 1 cup heavy cream with 1/4 cup sugar until stiff peaks form.

Pipe or spoon the whipped cream onto the top of the pie or onto individual slices before serving.