1. If using a ready-made graham cracker crust, skip to step #2. Heat the oven to 375ºF/180C. In a mixing bowl, combine the graham cracker crumbs and 1/4 cup of the sugar. Add 4 tablespoons of the butter, rubbing it in with a pinching motion until evenly distributed. Press the crumb mixture into a 9-inch pie pan and bake for 8 to 10 minutes or until golden brown. Allow the crust to cool.
2. Beat the egg yolks in a medium bowl and set them aside. Combine the remaining cup of sugar, cornstarch, cocoa powder, and salt in a 2-quart saucepan. Set the pan over medium heat and gradually whisk in the milk, stirring until smooth. Bring the mixture to a boil, stirring often. Boil for 1 minute, stirring constantly, then remove the pan from the heat.
3. Whisk 1/2 cup of the hot chocolate mixture into the egg yolks. Repeat with 1 cup of the chocolate mixture, then return the chocolate and egg mixture to the saucepan. Boil over medium heat for 1 minute, stirring constantly.
4. Remove the pan from the heat and stir in the remaining 2 tablespoons of butter and the vanilla extract. Pour the filling into the pie shell and allow it to cool at least 15 minutes, or chill it in the refrigerator overnight.
5. Just before serving, heat the oven to broil. Cover the pie with the mini marshmallows and broil it on the uppermost rack for about 30 seconds or until the marshmallows are lightly browned. (Watch carefully, or the marshmallows will burn.) Serve warm or chilled. Serves 8.