MAKE THE CRUST: Pulse the pretzels in a food processor until finely ground. In a large bowl, combine the pretzel crumbs, butter & brown sugar. Mix with your hands. Press the mixture into the bottom & up the sides of a 9" pie plate. Cover & refrigerate.
MAKE THE FILLING: Combine the cream cheese, peanut butter & brown sugar in a medium bowl & beat with a mixer until smooth. Fold in the whipped cream. Spoon the filling into the pie shell, cover & return to the refrigerator.
MAKE THE CHOCOLATE WHIPPED CREAM: Put the chocolate & whipping cream in the top of a double boiler over medium heat. Whisk until melted. Cover & refrigerate for 2 hours. Once cooled, beat the chocolate cream with a mixer until thick & spreadable (do not overmix).
Cover the pie with the chocolate whipped cream. Garnish with chocolate shavings if using. Serve the pie right away or refrigerate for another 2 hours.