Preheat oven to 350°F. Butter 9-inch pie dish.
In food processor, pulse cookies and pecans until reduced to crumbs. In medium bowl, combine cookie and nut mixture with sugar, salt and butter. Press into prepared pie dish and bake until crust is golden, 8-10 minutes. Let cool completely.
To make filling, whisk together 1/2 cup milk with cornstarch. Set aside.
In heavy bottomed saucepan, place sugar over medium heat. Let sugar melt and caramelize, swirling pan if necessary to make sure it caramelizes evenly. Keep close watch to make sure it doesn't burn.
When sugar has completely caramelized, remove from heat and slowly pour in remaining 3 1/2 cups of milk, whisk whisking constantly. Caramel will bubble and steam violently at beginning. Once all milk has been added, return pan to medium heat and whisk to dissolve any caramel that may have hardened. Boil for a few minutes until slightly thickened.
Slowly whisk in milk and cornstarch mixture, vanilla and salt and stir until pudding is thickened.
Remove from heat and cool over an ice water bath to room temperature. Alternately, refrigerate for 1 hour.
In bowl of electric mixer, beat heavy cream to soft peaks, and then fold it into cooled caramel mixture. Pour into pie crust and chill for 2 hours or more before serving. For cleanest slices, let the pie chill completely for at least 6 hours (I like to make mine the night before serving).
To serve, beat whipping cream, powdered sugar and vanilla to soft peaks. Spread on top of pie and top with chopped pecans.