Bake sweet potatoes at 350 degrees F for 1 hour or until very soft. Cool slightly. Cut potatoes in half; scoop out the pulp and discard shells. Place pulp in a food processor or blender; cover and process until smooth.
In a bowl, combine the pulp, brown sugar, flour, orange peel, pumpkin pie spice, vanilla and salt. Stir in milk and egg substitute until we blended. Pour into pastry shell.
Bake at 375 degrees F for 45-50 minutes or until a knife inserted near the center comes out clean. Cool on a wire rack for 2 hours. Garnish with whipped topping. Refrigerate leftovers.