Preheat the oven to 350 F. In a large bowl, mix together brownie mix, eggs, and vegetable oil until smooth. Use a cookie scoop to place the cookies onto a large ungreased baking sheet at least 2-inches apart.
Bake for 10-12 minutes. Remove half of the cookies from the baking sheet and set aside. Place marshmallows and chocolate chips on the remaining half of cookies. Bake for another 2-3 minutes. Remove from the oven, and sandwich with another cookie.