1. Preheat oven to 350°. Whisk together first 4 ingredients until well blended. Pour mixture into piecrust; place pie on a baking sheet.
2. Bake at 350° for 17 minutes or until pie is set. Remove pie from baking sheet to a wire rack, and let pie cool completely (about 1 hour).
3. Chill 3 to 24 hours. Top with whipped topping just before serving. Garnish, if desired.