Prepare Baked Pastry Shell. Set shell aside.
For The Baked Pastry Shell: Stir together flour and salt. Cut in shortening. Moisten dough with water, adding 1 tablespoon at a time and tossing with a fork. On a lightly floured surface, roll dough into a 12-inch circle.
Ease pastry into a 9-inch pie plate. Trim pastry to 1/2 inch beyond edge of pie plate. Fold under extra pastry. Crimp edge. Prick bottom and side of pastry. Line pastry with a double thickness of foil. Bake in a 450 degrees F oven for 8 minutes. Remove foil. Bake for 5 to 6 minutes more or until golden brown. Cool on wire rack.
For The Butterscotch Filling: In a medium saucepan, combine 1/2 cup of the brown sugar and the 1/4 cup butter. Cook and stir over low heat until butter melts and mixture is smooth. Remove from heat.
In a small bowl, combine the remaining 3/4 cup brown sugar, the flour and cornstarch. Add flour mixture to butter mixture; stir until combined. Gradually stir in whipping cream. Return saucepan to heat. Cook and stir over medium heat until thickened and bubbly; reduce heat. Cook and stir for 2 minutes more. Remove from heat.
In a small bowl, lightly beat egg yolks with a fork. Gradually stir about 1 cup of the hot filling into yolks. Add yolk mixture to saucepan. Bring to a gentle boil over medium heat, stirring constantly; reduce heat. Cook and stir for 2 minutes more. Remove from heat. Stir in the 3 tablespoons butter and vanilla. Pour filling into the Baked Pastry Shell. Cover surface of filling with plastic wrap. Chill in the refrigerator for at least 2 hours before serving.
To serve, prepare Whipped Cream Topping. Remove plastic wrap covering filling. Spread topping evenly over cold filling. (Or, if you like, spoon Whipped Cream Topping into a decorating bag fitted with a star tip. Decoratively pipe topping over filling.) Immediately serve pie or store, covered, in the refrigerator until ready to serve.
To Make Whipped Cream Topping: 1.In a clean mixing bowl, beat whipping cream with an electric mixer until soft peaks form (tips curl). Add powdered sugar and vanilla. Beat the topping mixture until stiff peaks form (the tips stand straight).