If you like buttered rum-raisin ice cream, you'll love this pie. We've used dark rum for rich flavor, but light rum will work fine. The pie filling should be cooled completely before the whipped topping is added.
Prepare and bake Piecrust in a 9-inch pie plate. Cool completely on a wire rack.
Combine raisins and rum in a small microwave-safe bowl; microwave at HIGH for 1 minute or until raisins are plump. Set aside.
Combine the sugar, cornstarch, salt, and eggs in a large bowl, and stir well with a whisk.
Heat milk over medium-high heat in a small, heavy saucepan to 180° or until tiny bubbles form around edge (do not boil). Gradually add hot milk to sugar mixture, stirring constantly with a whisk. Place mixture in pan; cook over medium heat until thick and bubbly (about 10 minutes), stirring constantly. Remove from heat; stir in butter.
Spoon custard into a bowl; place bowl in a large ice-filled bowl for 10 minutes or until custard comes to room temperature, stirring occasionally. Remove bowl from ice. Stir in raisins and vanilla; spoon mixture into prepared crust. Spread whipped topping evenly over filling. Loosely cover and chill 8 hours or until firm.