In a medium sized bowl, mix together flour, baking soda, and salt.
In a stand mixer or bowl for a hand mixer, cream together butter, peanut butter, brown and granulated sugar until creamy.
Beat in egg and vanilla into the sugar butter mixture until well combined.
With the mixer on low, slowly add in the dry ingredients to the wet. Once combined mix in marshmallows and chocolate chips.
Use a ice cream scoop or dough scoop to scoop dough into balls and place on a cookie sheet. Chill for 1 hour.
After chilling, space cookies on a sheet and bake for 12-14 minutes at 350°.
Let cool completely on the baking sheet and then Enjoy!