In a large bowl, combine melted butter and 2 1/2 cups graham crumbs, mixing well until moistened. Press into an 8 or 9-inch springform pan, evening the crust out with the back of a spoon. Place in the freezer to set.
While crust is setting, heat milk, sugar and chocolate in a saucepan over medium heat. Bring to a boil then reduce to a simmer and stir continuously for 20 minutes until fudge sauce is thickened. Remove and let cool for 15 minutes.
About 15 minutes before you remove the crust, take ice cream out of the fridge and allow to softened. Once soft, press and spread on top of the fudge sauce - don't worry, it may mix together a bit! Not a big deal. Smooth ice cream top and place back in the freezer for at least 2 hours, or even over night.
Before serving, toast marshmallows as desired. I placed mine on a baking sheet under the broiler for 1-2 minutes, then removed them with a spatula. Remove cake from freezer and sprinkle ice cream top with half of the remaining graham crumbs, then top with marshmallows. Freeze for another hour or so, then sprinkle with remaining graham crumbs. Serve!