Make the Crust: Preheat oven to 325 degrees F. Lightly grease a 9-inch springform pan.
In a medium bowl, stir together the graham cracker crumbs, sugar and melted butter until evenly moistened and totally combined. Press onto the bottom and 1 inch up the sides of the springform pan. Place the pan on a rimmed baking sheet and bake for 10 minutes. Remove to a wire rack while you prepare the rest of the cheesecake.
Make the Cheesecake: Combine the chocolate with ¾ cup of the evaporated milk in a heatproof bowl and place over a small saucepan of barely simmering water. Stir occasionally until chocolate is completely melted and the mixture is smooth. Pour the chocolate mixture into the crust and place in the freezer until set, 20 to 30 minutes.
Using an electric mixer, beat the cream cheese on medium speed until completely smooth, 2 to 3 minutes. Add the remaining evaporated milk, marshmallow creme, cornstarch and vanilla extract, and beat until combined and smooth, about 2 minutes. Reduce the mixer speed to low and beat in the eggs one at a time, beating for about 15 seconds before adding the next. Using a rubber spatula, mix a few times by hand to ensure that all of the ingredients are evenly incorporated. Pour the filling over the chocolate layer.
Place the springform pan back on the rimmed baking sheet and place in the oven. Bake until the outer couple inches of cheesecake looks puffed and set, but the center still jiggles when you move the pan, about 65 to 75 minutes. Sprinkle the top of the cheesecake with the marshmallows, return the pan to the oven and increase the temperature to 400 degrees F. Bake until the marshmallows are golden brown, an additional 6 to 8 minutes.
Turn off the oven, crack open the door and leave the cheesecake inside for 1 hour. After an hour, remove the cheesecake from the oven, place on a wire cooling rack and run a thin knife around the edge of the cake. Allow to cool completely to room temperature. Once the cheesecake has cooled to room temperature, refrigerate for at least 4 hours, preferably 8 hours or overnight. Leftover cheesecake can be stored in the refrigerator for up to 1 week.