In a large saucepan, combine custard mix, brown sugar, and pumpkin pie spice. Mix well, then add pumpkin, condensed milk, and egg. Whisk mixture until smooth, then cook over medium heat, stirring constantly, until mixture comes to a boil. Boil and stir 1 minute.
Pour mixture into baked pastry shell and chill. Garnish with whipped topping before serving.