I Made the Pioneer Woman Chocolate Pie and It's a Slice of Heaven

Name: I Made the Pioneer Woman Chocolate Pie and It's a Slice of Heaven
Cuisine: American
Ingredients: 1 whole pie crust, baked and cooled (or you can use Oreo or graham cracker crust); 1 1/2 cup sugar; 1/4 cup cornstarch; 1/4 teaspoon salt; 3 cups whole milk; 4 whole egg yolks; 6 1/2 ounces bittersweet chocolate, chopped finely; 2 teaspoons vanilla extract; 2 tablespoons butter; Whipped cream, for serving