Preheat the oven to 350°F. Grease a 9×13” baking dish and set aside. In a medium bowl, whisk together the flour, salt, baking powder, and cinnamon. Set aside.
In a large mixing bowl, cream together the butter and brown sugar until smooth and fluffy, about minutes. Beat in the eggs and vanilla until well blended. Mix in the flour mixture, 1/2 cup chocolate chips, 1/2 cup butterscotch chips, and mini marshmallows until combined. Spread the batter into the prepared pan and sprinkle evenly with the remaining chocolate chips and butterscotch chips.
Bake for 35–40 minutes, until the top is golden brown and the sides are starting to pull away from the edges of the pan. Let cool completely before serving!
These bars freeze great. Let them cool completely, cut, and freeze in a freezer bag or airtight container for up to 3 months. Let thaw on the counter for a few hours or overnight before serving!