Preheat oven to 350°F and grease a standard 24-cup mini muffin tin well.
Beat butter and both sugars, then add egg and vanilla and mix until smooth.
Stir in flour, graham cracker crumbs, baking soda, and salt to form a dough.
Roll dough into 24 balls, press each into a mini muffin cup, and bake for 10 to 12 minutes until set.
While still warm, use a small spoon to press an indentation into each cup, then let cool.
Heat heavy cream until hot, pour over chocolate chips, let sit 2 minutes, then stir into smooth ganache and spoon into each cup.
Top each cup with a marshmallow and broil for 1 minute until toasted.