Special equipment: a kitchen torch
Combine the 1/3 cup peanut butter, confectioners' sugar and heavy cream in a medium bowl and beat with an electric mixer until light and creamy, about 2 minutes. Spoon the mixture into the bottom of the pie crust and smooth evenly. Freeze until slightly firm, about 20 minutes.
Spoon the softened ice cream on top of the peanut butter cream and spread to fill the pie crust. Freeze until very firm, at least 2 hours.
Mound the marshmallows in the center of the pie and torch them with a kitchen torch; you can also broil them in the oven, but watch them carefully if you do. Warm the remaining 2 tablespoons peanut butter in the microwave until pourable, about 30 seconds. Drizzle the chocolate syrup and peanut butter over the pie. Freeze until ready to serve.
Propane gas torches are highly flammable and should be kept away from heat, open flame and prolonged exposure to sunlight. They should be used only in well-ventilated areas. Follow torch manufacturer's instructions for use.