Preheat oven to 350 degrees. Line a 13×9 baking dish with parchment paper.
In small bowl, melt butterscotch chips with butter in microwave, at 30 second increments, stirring between each time. Set aside.
In large mixing bowl, add brown sugar, eggs, and vanilla. Mix in melted butterscotch chips, flour, baking powder and salt. Fold in marshmallows and chocolate chips. Pour batter into baking dish.
Bake for 18-22 minutes. Cool, cut into squares and refrigerate for one hour. Enjoy!
Notes
Parchment Paper. Definitely use it. It will keep these gooey, sticky bars from sticking to the pan. Don't accidentally use foil or wax paper, won't have the same results.
Storage. These bars taste best when chilled, but can be stored in airtight container at room temperature.
Freeze. You can freeze these bars by wrapping in plastic wrap then sliding into freezer safe ziploc or airtight container. Freee for up to 3 months.
Substitutions. Swap out the butterscotch morsels for peanut butter morsels. You're welcome!