Preheat the oven to 350°F.
For the crust: In a large bowl, mix together the graham cracker crumbs, butter, and sugar until well combined. Firmly press the mixture into an ungreased 9-inch deep-dish pie plate. Bake for 5 minutes to set the crust. Remove, and let cool while you prepare the filling.
For the filling: In a medium saucepan, stir together the sugar, cornstarch, and salt. Add the milk and the beaten egg yolks. Cook, stirring, over medium-high heat until the mixture comes to a boil and thickens. Remove the pan from the heat, and add the chopped chocolate, butter, and vanilla, stirring until the chocolate has melted and is fully incorporated.
Pour the mixture into the pie crust, and smooth the top. Refrigerate until completely set, about 4 hours.
When ready to serve, arrange the marshmallows on the pie, and use a kitchen torch to toast the marshmallows. Alternatively, place the pie under the broiler until the marshmallows are toasted, 60 to 75 seconds. Using a vegetable peeler, shave some of the chocolate bar over the pie. Slice and serve.