Combine 1 cup flour and 1/8 teaspoon salt in a bowl; cut in shortening with a pastry blender until mixture resembles coarse meal. Sprinkle ice water, 1 tablespoon at a time, over surface of flour mixture; toss with a fork until dry ingredients are moistened. Gently press dough into a 4-inch circle on heavy-duty plastic wrap; cover with additional plastic wrap. Roll dough, still covered, into an 11-inch circle. Place dough in freezer 5 minutes or until plastic wrap can be easily removed.
Remove plastic wrap; fit dough into a 9-inch pieplate coated with cooking spray. Fold edges under, and flute; prick bottom and sides of dough with a fork. Bake at 425° for 15 minutes or until lightly browned; cool completely on a wire rack.
Combine 2/3 cup sugar, 2/3 cup flour, and 1/8 teaspoon salt in a medium saucepan. Gradually add milk and egg yolk, stirring with a wire whisk until well blended. Place over medium heat, and cook 16 minutes or until thickened and bubbly, stirring constantly. Remove from heat; stir in margarine and vanilla. Pour mixture into prepared crust; cover with plastic wrap. Chill 4 hours or until set.
Remove plastic wrap. Dollop whipped topping over filling; sprinkle each dollop with brown sugar.