Make the crust by stirring together the crushed pretzel, flour, and sugar in a mixing bowl. Add in the melted butter and stir until combined. Press the crust mixture into a 9 inch pie pan. Bake at 350 degrees for 10-12 minutes. Remove from oven and let cool.
Pour 1 cup hot fudge sauce into the baked, cooled pie shell. Spread out to cover the bottom and smooth. Set aside.
Combine sugar, cornstarch, milk, cream, peanut butter chips, and peanut butter in a medium saucepan. Cook over medium heat, stirring constantly, until mixture begins to bubble. This will take several minutes. Once it bubbles continue stirring for 2 minutes. The mixture will begin to thicken.
Mix a little of the hot mixture into the egg yolks (do this part quickly), beating quickly to avoid cooking the yolks with the hot liquid. Stir the warm yolk mixture back into the remainder of the peanut butter mixture and cook for an additional 90 seconds. Remove from heat and stir in the butter and vanilla and continue stirring until butter is melted and mixture is smooth.
Pour peanut butter custard into the pie shell over the fudge sauce. Allow mixture to cool for 30 minutes before putting it in the refrigerator to finish cooling. Let it cool in the refrigerator for at least 2 hours before serving.
Extra fudge sauce can be drizzled over the top for decorating. Add some more crushed pretzel and a little shaved chocolate if desired. Just wait until the peanut butter custard cools before adding the drizzles and toppings otherwise you'll end up with puddles of chocolate.