Old Fashioned Chocolate Pie

Ingredients
1 cup sugar
2 tablespoons cornstarch
1/4 teaspoon salt
2 cups milk
2 ounces unsweetened chocolate, chopped
3 egg yolks, beaten
2 tablespoons butter
1 teaspoon vanilla extract
1 pre-baked pie crust
Whipped cream (for topping)

Directions