To make the ice cream, combine ¾ cup of the cream and ¾ cup of the milk in a small saucepan with the sugar. Heat over medium, stirring occasionally, until the sugar is fully dissolved. Add the mixture to a blender with the remaining ¾ cup cream, ¾ cup milk, and the toasted marshmallows. Puree until the mixture is smooth, about 30-60 seconds. Cover and refrigerate until well chilled.
Freeze the marshmallow ice cream base in an ice cream maker according to the manufacturer’s instructions. (If the mixture seems too thick to pour into the ice cream maker, blend it once more briefly to smooth it out.) Transfer to a storage container and freeze at least 1-2 hours to firm before proceeding with assembly of the cake.
To assemble the cake, line a 9 x 5-inch loaf pan with plastic wrap. Place one of the graham cracker slabs in the bottom of the pan. Add about one third of the toasted marshmallow ice cream to the pan and smooth into an even layer with a small offset spatula. Top with ½ cup of the hot fudge sauce and smooth to completely cover the ice cream. Transfer the pan to the freezer and let set at least 30 minutes before proceeding with the next layer. Repeat this same order two more times (graham cracker, ice cream, hot fudge sauce.) Once all the layers have been added, let chill thoroughly, at least 4 hours.
When ready to serve, turn the cake out of the pan and peel away the plastic wrap. Top with additional marshmallows, if desired, and use a kitchen torch to brown them. Use a serrated knife to slice the cake.
*You certainly don’t have to make your own graham crackers for this, but I promise it’s worth the effort. I found a half batch of this graham cracker dough to be just about right for the three layers needed here. If the shapes aren’t perfect rectangles or are a bit too big for the pan, don’t worry. Just cut them down to the appropriate size a few minutes after they come out of the oven.
If you choose to use premade graham crackers, just layer them in a manner to create a complete, even layer as indicated in the recipe.