Preheat oven to 350 degrees F. Grease a 9-inch pie dish with butter.
In a food processor, pulse the cookies and pecans until they are reduced to crumbs. In a medium bowl combine the cookie and nut mixture with sugar, salt and butter. Press into the prepared pie dish and bake until crust is golden, 8-10 minutes. Let cool completely.
To make the filling, whisk together 1/2 cup of the milk with the cornstarch. Set aside.
In a heavy bottomed saucepan, place the sugar over medium heat. Let the sugar melt and caramelize, swirling the pan if necessary to make sure it caramelizes evenly. Keep a close watch to make sure it doesn't burn.
When the sugar has completely caramelized, remove from heat and slowly pour in the 3 1/2 cups of milk whisk whisking constantly. The caramel will bubble and steam violently at the beginning. Once all the milk has been added, return the pan to the medium heat and whisk to dissolve any caramel that may have hardened. Boil for a few minutes until slightly thickened.
Slowly whisk in the 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, you can chill in the fridge for 1 hour.
Beat the heavy cream to soft peaks, and then fold it into the cooled caramel mixture. Pour into the pie crust and chill for at least 2 hours before serving.
To serve, beat whipping cream, powdered sugar, and vanilla to soft peaks. Spread on top of pie and top with chopped pecans.