Make the Graham Cracker Crust: In a large bowl, whisk together the graham cracker crumbs, malted milk powder, sugar and salt. In a measuring cup, whisk together the melted butter and heavy cream, then pour over the crumb mixture. Using a rubber spatula, fold and mix and the ingredients together until all of the dry ingredients are evenly moistened. Turn the mixture out into a 9-inch pie plate and press evenly along the bottom and up the sides of the pan. Refrigerate while the chocolate filling is prepared.
Make the Chocolate Filling: Place the chopped chocolate in a medium bowl. Combine the heavy cream and corn syrup in a small saucepan and place over medium heat. Warm the mixture until it is just barely boiling. Remove from the heat and pour over the chopped chocolate. Let sit for 1 minutes, then gently begin stirring with a rubber spatula (starting in the middle and working your way out), mixing until all of the chocolate is melted and combined. Add the butter and stir until melted and incorporated. (If the mixture cools down too much to totally melt the chocolate or butter, microwave in 10-second increments on 50% power, stirring until everything is completely melted together.)
Pour the chocolate mixture into the chilled pie crust and spread into an even layer. Gently press the marshmallows into the chocolate filling, covering the top of the pie with marshmallows. Chill the pie briefly (30 minutes to 1 hour) to allow the chocolate the set. When ready to serve, use a kitchen torch to toast the marshmallows. The pie can be kept at room temperature as long as it's not warmer than about 72 degrees. If your kitchen is warmer than that, then I would refrigerate the pie so the chocolate filling doesn't melt.
Storing: The pie can be kept at room temperature as long as it's not warmer than about 72°F. If your kitchen is warm, refrigerate the pie so the chocolate filling doesn't melt.
Making Ahead: You can easily make this pie up to 3 days in advance (I just can't promise you won't be tempted to eat it before you're ready to serve!). Make sure to save the torching of the marshmallows until you are planning to serve.