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 and brown sugar, mixing until smooth.
Add the egg, vanilla extract, baking soda, and salt. Stir until well combined.
Gently fold in the mini marshmallows and chocolate chips.
Using a tablespoon, scoop out dough and place it on the prepared baking sheet, spacing them about 2 inches apart.
Bake for 10-12 minutes or until the edges are lightly golden.
Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.