Preheat the oven to 350 degrees and prepare a baking sheet by lining with parchment paper. Set aside.
In a large bowl, use an electric mixer on medium speed to beat together the butter and sugar until light and fluffy.
Add the egg and vanilla, continue mixing until well incorporated. Add the chocolate chips and mix until fully combined. Set aside.
In a large bowl, whisk together the flour, baking soda and salt.
Slowly add the flour mixture to the butter, sugar, egg, vanilla and chocolate chip mixture. Lower the speed of the electric mixer and mix just until combined.
Stir in the marshmallows.
Use a 2 Tablespoon cookie scoop to shape dough balls. Place evenly apart on the prepared baking sheet.
Bake for 8-10 minutes or until slightly golden brown. Allow to cool for a few minutes before transferring to a cooling rack.