2 pounds sweet potatoes
3/4 cup packed brown sugar
1/4 cup all-purpose flour
2 teaspoons grated orange peel
1 teaspoon pumpkin pie spice
1 teaspoon vanilla extract
1/8 teaspoon salt
1 cup fat-free milk
1/2 cup egg substitute
1 (9 inch) unbaked pastry shell
1/2 cup reduced-fat whipped topping