Chocolate cookie crust
In a food processor, grind the cookies to a very fine powder. You should have about 1 1/2 cups. Put the crumbs in a bowl and stir in the sugar.
Pour the butter over the crumb mixture and mix until well combined. The mixture will feel wet. Turn the crumb mixture out into a 9-inch pie plate and press it into the bottom and up the sides. You can use the back of a large spoon to even out the crust.
Put the crust in the refrigerator while you make the filling.
Peanut butter filling with chocolate bottom
Melt the chocolate chips in a microwave oven or in a double boiler*(see below). Add the corn syrup and stir to combine. Use a spatula to spread the chocolate in a thin layer on the bottom of the chilled pie crust. Put the crust back in the refrigerator while you make the peanut butter filling.
Put the cream cheese, peanut butter, vanilla, and brown sugar in the bowl of an electric mixer fitted with the paddle attachment. Beat on medium speed until well combined and completely smooth. Set aside.
In a clean bowl, use the mixer fitted with the whisk attachment to whip the cream until soft peaks form. Remove the bowl form the mixer and, with a rubber spatula, gently fold the whipped cream into the peanut butter mixture until the mixture is uniform in color.
Pour the mixture into the prepared crust and freeze for at least 4 hours. Once the pie is frozen, you can cover it with aluminum foil and freeze for up to 3 days.
Easy fudge sauce
Place both chocolates in a medium heatproof bowl and set aside.
In a small saucepan, bring the cream to simmer over medium heat. Add the corn syrup and stir to combine. Remove from the heat and pour over chocolates. Let mixture sit for 2 minutes. Starting at the center of the bowl and working your way out to the edges, whisk the chocolate mixture in a circle until completely smooth. The hot fudge sauce, cooled to room temperature and covered tightly, will keep for 3 days in the refrigerator. Rewarm the sauce in a microwave oven on medium heat for 30 seconds, stir, and repeat until sauce is warm; or reheat it in a small saucepan over low heat.
Place a frozen piece of pie on a large serving plate and spoon 3 heaping tablespoons of the warm sauce directly over the top of the pie. Eat and enjoy immediately.
*Double boiler method
Fill a medium saucepan halfway with water, and place on stove over medium-low heat. Put the chocolate chunks or pieces in a medium or large metal bowl, and place the bowl on top of the saucepan. The bottom of the bowl should not touch the water. Stir the chocolate occasionally (about every minute) until chocolate is completely melted, shiny, and smooth. Do not overheat.