Creamy Chocolate Pie

1/2 cup water
1 tablespoon cornstarch
1 (14 ounce) can chocolate sweetened condensed milk
3 egg yolks, slightly beaten
2 tablespoons butter or margarine
1/2 teaspoon vanilla extract
1 graham cracker pie crust
Whipped topping

Stir together water and cornstarch. In saucepan, combine sweetened condensed milk and egg yolks; stir in cornstarch mixture. Cook over medium-low heat, stirring constantly until thick and bubbly, about 12 to 15 minutes.

Remove from heat; stir in butter and vanilla extract. Spoon into pie crust. Chill 3 hours or until set.

Top with whipped topping. Refrigerate leftovers.