Preheat oven to 400 degrees then back to 350 for bars and line 2 large baking sheets with a silpat liner or parchment paper. I used the half sheet size.
Place marshmallows into the center of your baking sheet and bake for about 10 minutes until nice and golden on the tops and puffed. Remove and reduce oven’s temperature to 350.
Place cake mix, butter, oil, egg, chocolate chips and quartered Tagalongs into a large mixing bowl. Stir to combine. Stir in toasted marshmallows, only stir to combine gently, you don’t want to mix too much. Transfer dough to prepared baking sheet and press evenly, don’t spread to edges, just within the border is fine. Bake for 18-22 minutes, until baked through. Remove and let cook for 30 minutes before slicing into squares. Serve and Enjoy!
You can substitute chopped Reese's Peanut Butter Cups in place of the Tagalong Cookies if desired.