Combine first 3 ingredients in a saucepan; gradually stir in milk. Cook over medium heat, stirring constantly, until mixture comes to a boil. Cook 2 minutes. Remove from heat. Gradually stir about 1/4 of hot milk mixture into beaten egg; add to remaining hot mixture, stirring constantly. Cook over medium heat, stirring constantly, 3 minutes or until thickened. Remove from heat; stir in butter, lemon rind, and lemon juice.
Set saucepan in a bowl of ice water. Cool completely (about 5 minutes), stirring often. Pour mixture into crust. Cover and chill 4 hours. Spread whipped topping over pie before serving.