Preheat oven to 400 degrees F. for marshmallows. (When up to the bars, put oven down to 350) Line 9x13 pan with nonstick foil.
Place marshmallows on parchment paper lined sheet and bake for about 7 minutes until nice and golden and puffy. Remove and reduce oven’s temperature to 350.
Place cake mix, butter, oil, egg, chocolate chips and Tagalongs in bowl. Stir to combine. Stir in toasted marshmallows. Pat dough into pan and bake about 25 minutes until golden.