Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large mixing bowl, combine the peanut butter, granulated sugar, egg, vanilla extract, baking soda, and salt. Mix until smooth and well combined.
Gradually fold in the graham cracker crumbs and mini chocolate chips until evenly distributed.
Using a tablespoon, scoop out dough and roll it into balls. Place them on the prepared baking sheet, spacing them about 2 inches apart.
Gently flatten each ball with the back of a fork or your hand.
Bake for 10-12 minutes, or until the edges are lightly golden.
Remove from the oven and let them cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
Once cooled, spread a generous layer of marshmallow fluff on the bottom side of half the cookies. Top with the remaining cookies to create sandwiches.
For best results, let the cookies sit for about 30 minutes to allow the marshmallow fluff to set.