Aunt Esther's Butterscotch Pie

2 `/2 cups waer
2 2/3 cup golden brown  sugar, NOT packed
2 tbsp butter
4 egg yolks, beaten
1 tsp Vanilla
4 tbsp Cornstarch
1 ea 9" pie shell, baked
Cool Whip for topping

in a double boiler whisk together brown sugar and cornstarch.  Add enough of the water to form a pqste.  Mix in the beraten egg yolks and add the remaining water.  Stirring constantly until thick, Remove from  heat.  Stir in butter.  Pour into pie shell.  Chill.  Top with Cool Whip