chocolate pie recipe

1 pre-made pie crust (you can use a graham cracker crust or a regular pastry crust) - 1 cup granulated sugar - 1/3 cup unsweetened cocoa powder - 1/4 cup cornstarch - 1/4 teaspoon salt - 2 3/4 cups milk - 3 large egg yolks, lightly beaten - 2 tablespoons unsalted butter - 1 teaspoon vanilla extract - Whipped cream (optional, for topping) - Chocolate shavings or cocoa powder (optional, for garnish)